It appears a Christian, Richard Zetter, has had an oversight here and misunderstood the Verse. He contends the Verse teaches there's a promise of Paradise in the Gospel and the Torah for those who kill and are killed for Allah. He states:
This verse promises Paradise for those who kill and are killed, fighting in the cause of Allah. It claims that such a promise is to be found in the Gospel.
I've looked into this and I don't really see this as a theme in the commentaries. In fact the translators don't seem to indicate this either. They seem to indicate the Verse teaches God promises Paradise to those who shun the ephemeral (by submitting one's will to God).
It is explicitly stated in Maulana Muhammad Ali's commentary that the Promise mentioned in the previous Scriptures is not concerning fighting/killing for God:
It should be borne in mind that the words they fight in Allah’s way, so they slay and are slain, are not a part of the promise, but are expressive of the condition of the Companions, and show that they were true to their promise. The promise to spend one’s person and property may be carried out in various ways under different circumstances, and the Companions of the Holy Prophet were as true to this promise during the thirteen years at Makkah as during the ten years at Madinah.” [Source]
The Study Quran also appears to teach the Promise refers to the purchase by God of the souls and wealth of the believers (it does not talk about killing or being killed).
Nouman Ali Khan states the Promise includes all Muslims and is not specifically concerning those who kill or are killed for God - again indicating he would not be expecting the previous Scriptures contain a Promise of Paradise for those who kill and/or kill for God but rather a Promise for those who have given up the world for devotion to Allah. Concerning those who kill/killed for God in just warfare, Nouman Ali Khan states this is the highest manifestation of this Promise but every Muslim is included in the Promise thus it is not confined to people on the battle field fighting for what is right.
Richard used the Sahih International translation and perhaps got a little confused but I will furnish other translations which are perhaps discern the Promise (underlined) from the statement concerning those who kill or are killed thus indicating they do not believe the Verse teaches the part about killing and being killed for God is in the Torah and the Gospel:
BEHOLD, God has bought of the believers their lives and their possessions, promising them paradise in return, ,[and so] they fight in God's cause, and slay, and are slain: a promise which in truth He has willed upon Himself in [the words of] the Torah, and the Gospel, and the Qur'an. And who could be more faithful to his covenant than God? Rejoice, then, in the bargain which you have made with Him: for this, this is the triumph supreme! [Muhammad Asad trans of Q 9:111]
GOD has bought from the believers their lives and their money in exchange for Paradise. Thus, they fight in the cause of GOD, willing to kill and get killed. Such is His truthful pledge in the Torah, the Gospel, and the Quran - and who fulfills His pledge better than GOD? You shall rejoice in making such an exchange. This is the greatest triumph. [Rashad trans. of Q 9:111]
God has purchased from the believers their persons and their goods, for (in return) theirs is the Garden. They shall fight in the cause of God, and shall slay and be slain. It is a promise that is binding on Him in the Torah and the Gospel, and the Qur’an. Who can fulfill a promise better than God? Rejoice then in the bargain you have made, for that is the Supreme Triumph. [Shabir Ahmed trans. of Q 9:111]
There's extra significance to Shabir Ahmed's translation as it carries a footnote in which he does offer Biblical verses which he believes Q 9:111 references. Interestingly enough he does not try and offer anything about killing or being killed for Paradise but rather verses about willingly submitting one's will to God. He cites Deut 6:4-5:
4 Hear, O Israel: The Lord our God, the Lord is one.[a] 5 Love the Lord your God with all your heart and with all your soul and with all your strength.
In addition to this there are other reference I reckon he cites from the NT, Matthew. I can't be 100% here as the formatting leaves it uncertain but you can check it for yourself. I think it's reasonable to assume he refers to Matthew 6:32-33 which I think is an instruction to prioritize God and the Hereafter over everything worldly.
32For the pagans run after all these things, and your heavenly Father knows that you need them. 33But seek first his kingdom and his righteousness, and all these things will be given to you as well.
Here's another I think he references. This seems to be close to the money, a Promise to compensate, with Paradise, those who have sacrificed this world for God. Matthew 19:29:
29And everyone who has left houses or brothers or sisters or father or mother or wife or children or fields for my sake will receive a hundred times as much and will inherit eternal life.
I also think he refers to Matthew 10:39 which I believe Christians would interpret as whoever shuns this life will get the Hereafter (ever lasting life):
39Whoever finds their life will lose it, and whoever loses their life for my sake will find it.
Syed Abul A'ala Maududi in his commentary talks about what the Promise is (again note the absence of talk concerning being killed/killing):
In this verse that aspect of the Islamic faith which determines the nature of the relationship between Allah and His servants has been called a transaction. This means that faith is not merely a metaphysical conception but is, in fact, a contract by which the servant sells his life and possessions to Allah and in return for this accepts His promise that He would give him the Garden in the life after death
Then he discusses parts of the Hebrew Bible and the New Testament which allude to the Promise (notice he's not looking for verses about fighting or martyrdom) but rather a renouncement of the lower life in submission to God for the Eternal Life:
Some critics say that the statement of promise in the Torah and the Gospel is not confirmed by these Books. Their objection in regard to the Gospel is obviously wrong for even in the existing Gospels there are sayings of Prophet Jesus (peace be upon him) that confirm this verse. For instance:
“Blessed are they which are persecuted for righteousness’ sake: for theirs is the kingdom of heaven.” (MAT. 5: 10).
“He that findeth his life shall lose it: and he that loseth his life for my sake shall find it.” (MAT. 10: 39).
“And every one that hath forsaken houses, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for my name’s sake, shall receive an hundredfold and shall inherit everlasting life.” (MAT. 19: 29).
It is, however, true that the matter of this transaction is not confirmed in its entirety by the existing Torah. For instance, there is a mention of the first part of the bargain at several places in one forth or the other: “Is not He thy father that hath bought thee? hath He not made thee, and established thee?" (DEUT. 32: 6).
"Hear, O Israel: The Lord our God is one Lord: And thou shalt love the Lord thy God with all thine heart, and with all thy soul, and with all thy might." (DEUT. 6: 4-5).
But as regards the other part of the bargain, that is, the promise of the Gardens, they applied it to the land of Palestine:
“Hear therefore, O Israel, and observe to do it; that it may be well with thee, and that ye may increase mightily, as the Lord God of thy fathers hath promised thee, in the land that floweth with milk and honey.” (DEUT. 6: 3).
This is because the Torah does not give any conception of the life-after-death, the Day of Judgment, rewards and punishments in the Hereafter, though this creed has always been an inseparable part of the right way. This does not, however, mean that the Torah did not originally contain this creed. The fact is that the Jews had become so materialistic during the period of their degeneration that they had no other idea of a reward from God than the wellbeing and prosperity in this world. Therefore they perverted all the promises made by God in return for man’s service and obedience to Him and applied those to the land of Palestine.
In this connection, it should also be noted that the above mentioned changes became possible because the original Torah had been tampered with in several ways. Some portions were taken away from it and others were added to it. Thus, the Torah in the existing form is not purely the word of God but also contains the comments, etc. of the Jewish scholars mixed up with it. So much so that at some places it becomes difficult to distinguish the word of God from the Jewish traditions, their racial prejudices, their superstitions, their ambitions and, wishes, their legal interpretations, etc. all of which have gotten mixed with the word of God. (See E.N. 2 of Aal-Imran).
Abdullah Yusuf Ali's commentary also indicates the Promise (that was in the Torah and the Injeel) is that concerning the covenant of redemption: one must surrender oneself completely in exchange for ever lasting life (Paradise). He goes on to say this surrender may include fighting for the cause, but again he does not seem to think the Promise mentioned in the previous Scripture was that concerning killing or being killed for God
C1362. We offer our whole selves and our possessions to Allah, and Allah gives us Salvation. This is the true doctrine of redemption: and we are taught that this is the doctrine not only of the Quran but of the earlier Revelations, -the original Law of Moses and the original Gospel of Jesus.
Any other view of redemption is rejected by Islam, especially that of corrupted Christianity, which thinks that some other person suffered for our sins and we are redeemed by his blood. It is our self-surrender that counts, not other people's merits.
Our complete self-surrender may include fighting for the cause, both spiritual and physical. As regards actual fighting with the sword there has been some difference in theological theories at different times, but very little in the practice of those who framed those theories...
...The monkish morality of the Gospels in their present form has never been followed by any self-respecting Christian or other nation in history. Nor is it common-sense to ignore lust of blood in unregenerate man as a form of evil which has to be combated "within the limits, set by Allah" (9:112). (R). [Source]
I think there's a lot here for Richard to consider, I trust he is sincere enough to look into this and make the relevant corrections if he is convinced by the evidence to do so.
There is no statement in Scripture that says, “God is three Persons in one being"
Notes from Sean Finnegan's interview with Patrick Navas: Is the Trinity Biblical