Sforno on Genesis 22:13/ספורנו על בראשית כ״ב:י״ג

1והנה איל אחר נאחז בסבך, this incident indicated to him that G’d must have arranged for this ram to be at his disposal, and that there could be no question of his committing robbery if he took this ram and sacrificed it as a burnt offering. The fact that there had been no such ram when he prepared the altar and bound Yitzchok and suddenly this ram materialised, was ample evidence of its purpose.

2תחת בנו, in exchange for what he had had in mind to do with his son. The phrase reminds us of Psalms 15,2 ודובר אמת בלבו, “he spoke truthfully in his heart.” [the author justifies the use of the word תחת, “in lieu of,” for something which had not actually happened, i.e. Yitzchok had not been slaughtered. The verse from Psalms proves that the intention is as good as the deed when such an intention was sincere, wholehearted. Ed.]