Who Takes Precedence -- Friend or Enemy?
Siftei Tzaddik, quoted in Piskei Teshuvot

Halakha often deals with the question, "What (or who) takes precedence?" The answer (as in the following selection) often reflects what the halakha considers high on its list of priorities. This short responsum appears in Piskei Teshuvot, quoting the Siftei Tzaddik.

Question
What should one do if presented with the following dilemma? Two people simultaneously approach you, each requesting to borrow the same object from you. One of them is a friend. The other, though, is someone who, when you requested to borrow from him, had refused you. Who takes precedence?

Answer:
There are two conflicting moral/halakhic principles, each one pointing in a different direction.

On the one hand, the prohibition against taking revenge dictates lending to the one who had previously refused you. The textbook Talmudic case of revenge is refusing to lend to someone who refused to lend to you, returning one affront with another.

On the other hand, the trait of appreciation ("hakarat hatov") would require you to lend to your friend, who undoubtedly had previously done numerous favors to you. His request to borrow from your is a good opportunity to repay a favor.

There might be a model for dealing with this question in the Gemara. The Gemara, in telling the rules of helping another load and unload an animal ("perika ute'ina"), asks what one should do if his friend's animal needs help being unloaded and, at the same time, his enemy's animal needs help being loaded. The problem is compounded because helping an animal unload involves alleviating the animal's pain ("tzaar baalei chayim"). That would point towards helping the friend. The Gemara concludes that one should, in such a situation, help his enemy. The reason it gives is that involves subjugating one's evil inclination.

Similarly, here, it would be appropriate to lend to the one who had previously refused you, in order to subjugate your evil inclination. That would take precedence over appreciation, similar to the way subjugating the evil inclination takes precedence over alleviating pain to animals.

He ends with a quote from Sefer Chassidim that bolsters this. it says there that a mitzva that involves subjugating one's evil inclination is worth a hundred other mitzvot.

Note from Hagaon Rav Avraham Mordechai Alter zt"l, the Gerrer Rebbe, on another selection from the Siftei Tzaddik. The Siftei Tzaddik had applied the same principle to the tragic conflict where two people approach a third asking for water. There is not enough water for both to drink and live. If one is his enemy and the other his friend who should he give to? The Siftei Tzaddik applied the Gemara's rule and said to give to the enemy. The Gerrer Rebbe objects that we do not find this principle, that an enemy takes precedence over a friend, applied to areas outside the rules of loading and unloading.