Spoils and Tzedaka

The אור החיים הקדוש asks an interesting question. On the one hand, he is diligent in giving מעשר, his tithe, to the Kohein, who was actually שם בן נח. However, when he was offered spoils of war after saving Lot, he refused taking a thread ( a hint to ציצית), or a shoe lace ( a hint to the תפילין straps). It seems strange that he won’t take spoils, but he continued to give the Kohein, who was also one of the kings (מלכי צדק). The אורח חיים explains that the מעשר was Avraham’s obligation. That is a constant that does not change. But when it comes to his taking money from them, he refuses. He felt there were strings attached, and he didn’t want them boasting that they made Avraham rich. Therefore, it is not a contradiction.