Why does Abraham debate with G-D over the destruction of Sodom & Gomorrah but offers no plea when commanded to offer his son Isaac?
I think that the answer lies in the way you phrased your question – “but offers no plea when commanded to offer his son Isaac.” This is precisely the point – Abraham was commanded to offer his son, and he would never argue with a direct command. On the other hand, when G-d decided to destroy Sodom, he did not command Abraham to do anything.
In fact, He only informed Abraham about His intentions out of a sense of love and respect, as one “Friend” sharing with another. We see this from G-d’s soliloquy just before His conversation with Abraham: “Shall I hide from Abraham that which I am about to do… I love him…”.
The Midrash explains that G-d could not “justify” destroying cities within the Land of Israel without first informing the “landlord” – the man to whom He had just given the land as a gift. Abraham had every right to debate, and it seems as though G-d was intentionally giving him the opening to do just that.
We see a similar scenario with another man famous for his debates with G-d – Moses. In response to building of the Golden Calf, G-d says to Moses (Exodus 32:10) “And now, just leave me alone… and I’ll destroy them [the Jewish People].” The commentators ask: Why would G-d tell Moses to leave Him alone if Moses had not even started to pray for the salvation of the Jews? The answer, given by our sages, is that G-d was hinting to Moses that the survival of the Jews depended on his prayers, and that only if Moses ‘left G-d alone’ would the Jews be destroyed.
Yours sincerely,
Ari Lobel