Having instructed Avimelech to return Sarah, why did HaSh-m see fit to add the fact that he is a Navi?
Rashi: To reassure him that Avraham would know that he had not had relations with Sarah, and therefore, a. Sarah would therefore not be rejected by him, and b. Avraham would willingly Daven on his behalf. 1
Gur Aryeh: What is Rashi explaining? The verse continues, "... for he is a prophet;" had Avraham not been a prophet, or had he declined to pray, would Avimelech have been exempt from returning his wife? Rather, it means he will know prophetically that she was not defiled.
Rashi (to Shemos 7:1) explains the etymology of the word "Navi" (prophet) as derived from "Niv Sefasayim" (speech, utterance). If so, why would our verse need to emphasize Avraham's status as a Navi - a speaker?
Gur Aryeh (to Shemos 7:1): The word Navi refers [initially] to someone who speaks the Word of HaSh-m specifically (which is hidden, and needs to be spoken publicly). 1 Our verse means, 'Return his wife, for he publicizes the Word of HaSh-m - and he will punish you if you fail to do so.
If so, the word 'Navi' in that context ("Aharon your brother shall be your Navi" (Shemos 7:1)) is a borrowed usage. (EK)
Why does the verse connote that Avimelech will die if Avraham does not pray for him?
Sha'arei Teshuvah (R. Yonah) 1:44: If one pained another, HaSh-m does not pardon him until he asks forgiveness from the one that he pained.
Moshav Zekenim: Avimelech was Chayav Misah. Avraham's Tefilah would save him - HaSh-m decrees, and Tzadikim annul His decree.
