Rashi says that "raising his hand" is an expression of a Shevu'ah. Why did Avraham need to swear?
Chochmah u'Musar (Vol. 2, p. 326, 371): According to letter of the law, Avraham acquired it all through conquest in war, especially since there was despair (Yi'ush) and Shinuy Reshus. He returned it for Kidush HaSh-m. In spite of all his Tzidkus and hating unjust gain, he did not rely on himself, and swore.
QUESTIONS ON RASHI
Rashi writes: "I have raised my hand - ... [At times, past tense indicates the present, such as,] 'I have given the money for the field' (23:13); which means, 'I am now giving.'" But in that verse, Rashi explains, "If only I had given it already," in past tense?
Gur Aryeh: In that verse, Avraham is not giving the money yet, rather, he is explaining how much he was willing to give the money currently (present tense).