Qitzur Shulchan Arukh – 65:6
If is forbidden to pay the interest early or to pay it late. How is this?
Re’uvein wanted to borrow something from Shim’on, and he preceded [the loan] by sending him a gift explaining that it was in order that he [Shim’on] lend him money, or it was an excessive gift that of its own is just like he had explained that it was in order that he lend him, that is early interest.
If he borrowed from him and returned his money, and then sends him a gift [in appreciation] for the money which was left unused at his [Re’uvein the borrower’s] place, this is delayed interest.
Lending your brother money should not require inducement nor expectations of huge gratittude. It’s what you do to help each other out. Therefore, such payments are included as ribis (literally: increase) even if they do not intuitively seem like interest.
That said, we saw in 65:2 that unlike true interest, if the gift was given (in violation of halakhah), there is no obligation to repay it.