Why did Avraham use the word "Anochi," and not 'Ani'?
Seforno: He was saying that he (Avraham) undertook to show kindness and swear (even on behalf of his children) - but that Avimelech had not performed with him the kindness he professed to have performed, seeing as his servants had stolen from him (as the next Pasuk explains).
Oznayim la'Torah: Avraham took an oath, but he did not let Avimelech do so 1 - telling him that he believed him on his word, since it is forbidden even to cause a Nochri to swear by his god. 2
Oznayim la'Torah: Yaakov, at the end of Parshas Va'Yeitzei (31:53), did likewise with Lavan.
See Oznayim la'Torah, who explains as to why Avraham and Yaakov entered into their respective agreements, despite the fact that the Chachamim forbade entering into a partnership with a Nochri lest he end up swearing by his god.
