Musar su Esodo 22:18

The great sage Rabbi Moshe Nachmanides already alludes to the קרבן aspect of this meal when he writes as follows: "Perhaps Abraham knew that the part of the food offered first would disappear." One may infer from those words that just as Manoach experienced the sudden disappearance of his offering together with the angel (Judges 13,20), Abraham was aware of the procedure and expected the angels to disappear. It is quite clear that Manoach had not built an altar, he had only made use of an existing rock to offer an offering upon it. (ibid.) The "altar" mentioned there in the book of Judges refers to the Celestial Altar, the region the angel had descended from. This is why he had said to Manoach "do not detain me, since I cannot eat from your bread." The reason he could not do so was that it was פיגול, unacceptable in principle, seeing that one must not offer a sacrifice to one of G–d's angels or other agents. Offerings, even when acceptable, must be offered only to G–d directly. (Judges 13,16). We also have a specific verse in the Torah precluding the offering of sacrifices to an intermediary i.e. בלתי לה' לבדו, "only to G–d Himself" (Exodus 22,18)צ Assuming Abraham was aware of this, he could not have meant for this bread to be eaten, for he knew that angels do not eat. His intention had been that this bread would serve exclusively as a meal offering to G–d in recognition of the fact that G–d had appeared to him while he was awake and able to move around freely. Our sages in Bereshit Rabbah 48,4 take the view that G–d's appearance was in response to Abraham having circumcised himself. G–d reasoned that just as He appears to someone who builds an altar for him and offers a sacrifice (by allowing that sacrifice to be consumed by Heavenly fire), if someone performs circumcision on his own body, such an act certainly deserves to be acknowledged by G–d appearing to such a person. Abraham understood the fact that G–d had revealed Himself to him as an outpouring of G–d's bounty, as is normal when G–d accepts someone's offering. He now wanted to offer a real (additional) offering in order to draw down still more of G–d's blessings.