In Hebrew, there are two words for lending: one is lehashil, which means to lend an object, like a car or a hammer. The other word is lehalvot, which means to lend a sum of money. Why two different words? Rashi explains that there is an inherent difference between lending an object and lending money: if I lend you my book, I'd sincerely appreciate your returning the same exact book to me. But if someone lends you fifty dollars, they don't need the same exact banknotes in return; any fifty dollars will suffice.
By the same token, there is a major difference between doing someone a material favor and doing someone a spiritual favor. Material favors should be returned in kind, even though the giver should not expect remuneration of any kind, otherwise it's not a true favor. For example, if someone mowed your lawn when you went on vacation, it would be nice of you to reciprocate when he goes on vacation.
Spiritual favors are different. If someone teaches you about emuna or how to talk to Hashem in personal prayer, you should of course be grateful. But, instead of returning the favor, pass it on to others. Teach them about emuna and how satisfying it is to have your own personal relationship with Hashem. And in that way, you're doing a big favor for the person who first helped you.
Olam Chesed yibaneh - we can build a better world by doing favors for one another.