Parshas Tetzaveh commences with the commandment to light the menorah in the Beis Hamikdosh each evening. What is the purpose of this mitzvah; does Hashem need us to illuminate the darkness for Him? The Medrash (Shemos Raba 36, 2) answers this question with a mashal.
"A blind man was walking with a friend. The friend turned to the blind man and said, 'Hold on to me and I will lead you.' Once they entered their house, the friend asked the blind man to light a torch to illuminate the area for them, 'So that you will not feel indebted to me for leading you.' The man gifted with sight symbolizes Hashem. The blind man refers to Bnei Yisrael who "groped in darkness" when they committed the sin of the golden calf. Despite their transgression, Hashem continued to lead them through the desert with the pillar of fire. Once Bnei Yisrael began building the Mishkan, Hashem commanded Moshe to light the Menorah. This way, Bnei Yisrael would, so to speak, illuminate the Mishkan for Hashem just as He illuminated the way in the desert for them."
Rav Wolbe (Alei Shur vol. II pg. 281) quotes Rav Yeruchom Levovitz zt"l, who said that we can glean from this Medrash how to perform a perfect act of chesed. After helping out another person, the benefactor should ask the beneficiary for a small favor, since no one likes to feel indebted. Asking for a small favor will prevent the beneficiary from feeling indebted to the one who performed the chesed.
Often we assist others and decline any remuneration. For example, we might give them a ride and refuse to accept any payment. Whether or not we accept the payment, we have performed a mitzvah De'Oraisa of gemillus chasadim. Yet, while sometimes a complete chesed entails not accepting money, at other times a complete chesed necessitates accepting the payment.By accepting their money you are allowing them to express their hakaras hatov, thereby preventing them from feeling indebted to you in the future. Mi k'amcha Yisrael! Who else looks to do chesed without expecting a pat on the back?