Problem: Hebrews reads, “Therefore, when He comes into the world, He says, ‘Sacrifice and offering you have not desired, but a body you have prepared for me’” (Heb. 10:5). However, when we flip back to the original reference, we read, “Sacrifice and meal offering You have not desired; My ears You have opened” (Ps. 40:6). Critics claim that the author is blatantly twisting this verse.
Solution: A number of points can be made.
First, it would be odd if the author of Hebrews deliberately twisted the text. The purpose of the book of Hebrews is to use the OT to convince Jewish believers to abandon the OT sacrificial system. Why would the author of Hebrews appeal to an OT passage, if his opponents could catch him in such a clear act of deceit?
Second, this Hebrew idiom in Psalm 40:6 is difficult for anyone to translate. In fact, even orthodox Jewish interpretations (e.g. NJPSV, Stone Edition, etc.) have difficulty translating this passage. This is a Hebrew idiom which literally means “My ears You have opened…” However, many translators (both Jewish and Christian) have tried to interpret this obscure expression in different ways.
Third, the author of Hebrews had been quoting the Septuagint. The Septuagint translates Psalm 40 in this way, so the author of Hebrews used the same exact translation consistently throughout his entire book. In other words, he didn’t simply quote it when it benefited him. The Septuagint was widely acknowledged as a good translation at the time, so the author of Hebrews used it for this difficult expression.
The Septuagint translates this idiom as a form of synecdoche (where a part stands for the whole). A modern example of synecdoche would be if a teenager asked for the keys to the car. By asking for the keys, he is really asking for the car as a whole. By referring to the ears which hear the word of God, the Septuagintal translators understood this to mean David was really referring to the whole man (his “body”).
Fourth, the author of Hebrews’ argument does not hang on this part of the citation. The portion about a body being prepared does not carry the argument for the author of Hebrews. He bases his argument on the non-disputed portion of the text.
David wrote Psalm 40 according to the superscription. Remember, Israel’s first king (Saul) sacrificed animals, rather than following God’s will, and God took the kingdom away from him because of it (1 Sam. 15:22-23). Here, in Psalm 40, David is choosing to follow God’s will, rather than focus on sacrifices.
The author of Hebrews quotes Psalm 40 because it is in line with internalizing God’s law, rather than mere outward obedience. In the previous chapter, he quoted Jeremiah 31 for the same reason. Carson and Beale write,
The author uses this quotation to emphasize that the sacrifices of the old system were, by their nature, unsatisfactory in attaining God’s ultimate goal of relationship with a covenant people. The inadequacy of the sacrifices in and of themselves finds expression elsewhere in the OT (e.g., 1 Sam. 15:22; Ps. 50:8–11; Isa. 1:10–13; Jer. 7:21–24; Hos. 6:6), pointing rather to heartfelt devotion as the most foundational aspect of true worship. The practice of cultic rituals, apart from heart obedience, misses God’s intention for the sacrificial system.
OT scholar Michael Brown writes,
The Messiah, the only one who was completely obedient, the one who could truly say, ‘In the scroll of the book it is written about me,’ since he was the ultimate sacrifice, the perfect offering, the one who fulfills the image of the sacrifices of atonement and cleansing. His life satisfied the real meaning of the sacrificial system. He was not just the one who was totally yielded to the will of the Father; he was the one who actually offered himself up as a sin offering. So, according to Hebrews 10, when Jesus the Messiah came into the world… he said, ‘God, you don’t want more sacrifices and offerings. You have already received hundreds of thousands of lambs and goats and rams and bulls. You want me! I’m the one you spoke of in your Law’—and for the first time, the Scripture was fulfilled and the goal was realized.
 Brown, Michael L. Answering Jewish Objections to Jesus: New Testament Objections. Volume Four. Grand Rapids, MI: Baker, 2007. 39.
 Beale, G. K., & Carson, D. A. (2007). Commentary on the New Testament use of the Old Testament (p. 978). Grand Rapids, MI; Nottingham, UK: Baker Academic; Apollos.
 Brown, Michael L. Answering Jewish Objections to Jesus: Messianic Prophecy Objections. Volume Three. Grand Rapids, MI: Baker, 2003. 130-131.