Rashi says that lifting his hand is an expression of a Shevu'ah. Why did he need to swear?
Chachmah u'Musar (2 p.326, 371): According to letter of the law, Avraham acquired it all through conquest in war, especially since there was despair 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.
רש"י: נתתי כסף השדה...נותן אני לך כסף השדה: לקמ' (כג,יג) פירש רש"י על נתתי כסף השדה שזה בלשון עבר- "מוכן הוא אצלי והלואי נתתי", וכאן פירש שזה בלשון הווה?