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 Hashem. 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).