The inquirer says, “We often hear that it is recommended to reciprocate a favor or a gift with something similar.” What is the evidence for this in Islamic law?

Islam is a religion characterized by mercy, kindness, and maintaining good relations. For this reason, Allah Almighty enjoins justice and benevolence, making them among the highest values in Islam. He says, “Indeed, Allah commands justice and excellence”
(Quran, 16:90).

It was reported that the Messenger of Allah (peace and blessings be upon him) would accept gifts and reciprocate them. ‘A’isha (may Allah be pleased with her) said, “The Messenger of Allah, peace and blessings be upon him, used to accept gifts and reward people for them”
(recorded by al-Bukhari in his Sahih).

Abu Humayd al-Saʿdi (may Allah be pleased with him) narrated, “The messenger of Ibn al-ʿAlama`, ruler of Aylah, came to the Messenger of Allah (peace and blessings be upon him) with a letter and presented him with a white mule as a gift. The Messenger of Allah (peace and blessings be upon him) wrote back to him and gifted him a cloak”
(recorded by Bukhari and Muslim).

 ‘Abdullah Ibn ‘Umar (may Allah be pleased with both of them) narrated that ’Umar Ibn al-Khattab passed by a Jew with the Prophet (peace and blessings be upon him) who was wearing two shirts. The Jew said, “O Abu al-Qasim, clothe me.” The Prophet (peace and blessings be upon him) took off the better of the two shirts and gave it to him. ‘Umar asked, “O Messenger of Allah, why did you not give him the lesser one?” He replied, “O ʿUmar, do you not know that our true religion is free of stinginess? I gave him the better shirt so that he would feel more inclined toward Islam.”
This hadith was recorded by the hadith scholar, Abu Nuʿaym, in Hilyat al-Awliya`. This principle is further supported by the verse, “Is the reward for goodness anything but goodness” (Quran, 55:60).

Based on the above,
Islamic texts clearly encourage benevolence, maintaining good relations, replying to greetings in kind, reciprocating gifts, and responding to good deeds with goodness. These actions strengthen the bonds of brotherhood and harmony among people.

And Allah Almighty knows best.