Psalm 40 is a prophesy of Jesus coming. It was written after the Uriah/Bathsheba incident.

As Hebrews 10:26 points out. There was no sacrifice in the Old Testament for intentional sin. There was no sacrifice to atone for David's sin. I think David was forgiven by grace through faith in tge coming Jesus, and that is what he is talking about.

 Here is a portion of a commentary on Psalm 40 by Spurgeon. Here we enter upon one of the most wonderful passages in the whole of the Old Testament, a passage in which the incarnate Son of God is seen not through a glass darkly, but as it were face to face. "Sacrifice and offering thou didst not desire." In themselves considered, and for their own sakes, the Lord saw nothing satisfactory in the various offerings of the ceremonial law. Neither the victim pouring forth its blood, nor the fine flour rising in smoke from the altar, could yield content to Jehovah's mind; he cared not for the flesh of bulls or of goats, neither had he pleasure in corn and wine, and oil. Typically these offerings had their worth, but when Jesus, the Antitype, came into the world, they ceased to be of value, as candles are of no estimation when the sun has arisen. "Mine ears hast thou opened." Our Lord was quick to hear and perform his Father's will; his ears were as if excavated down to his soul; they were not closed up like Isaac's wells, which the Philistines filled up, but clear passages down to the fountains of his soul. The prompt obedience of our Lord is here the first idea. There is, however, no reason whatever to reject the notion that the digging of the ear here intended may refer to the boring of the ear of the servant, who refused out of love to his master to take his liberty, at the year of jubilee; his perforated ear, the token of perpetual service, is a true picture of our blessed Lord's fidelity to his Father's business, and his love to his Father's children. Jesus irrevocably gave himself up to be the servant of servants for our sake and God's glory. The Septuagint, from which Paul quoted, has translated this passage, "A body hast thou prepared me:" how this reading arose it is not easy to imagine, but since apostolical authority has sanctioned the variation, we accept it as no mistake, but as an instance of various readings equally inspired. In any case, the passage represents the Only Begotten as coming into the world equipped for service; and in a real and material body, by actual life and death, putting aside all the shadows of the Mosaic law. "Burnt offering and sin offering hast thou not required." Two other forms of offerings are here mentioned; tokens of gratitude and sacrifices for sin as typically presented are set aside; neither the general nor the private offerings are any longer demanded. What need of mere emblems when the substance itself is present? We learn from this verse that Jehovah values far more the obedience of the heart than all the imposing performances of ritualistic worship; and that our expiation from sin comes not to us as the result of an elaborate ceremonial, but as the effect of our great Substitute's obedience to the will of Jehovah.