Surah 74. Al-Muddaththir, Ayah 6

Lit. "and do not bestow favours to obtain increase".

 It was a common practice to give someone a gift, hoping to receive a more valuable gift in return. This practice is disliked in Islam.

The legal and commercial formula is that you give in order to receive. And usually you expect to receive what is worth to you a little more than you give. The spiritual consideration is that you give, but expect nothing from the receiver. You serve Allah and Allah's creatures.