Qitzur Shulchan Arukh – 180:14
When a borrower comes to pay the lender a debt that the shemittah [already] passed on it, the lender should say to him, “I retire the debt, and you are already absolved [of any obligation] from me.” If the borrower says to him, “Even so, I want that you accept it from me”, it is permitted for the borrower to accept the money from him.
And [in this situation,] the borrower should not say “I am giving this to you because of my obligation”, rather he should say to him “These [monies] are mine, and as a gift, I am giving them to you.” The lender may put in personal effort and work so that they borrower would say to him that he is giving the money as a gift. But if he can not accomplish this, he should not take the money.
As we said, a primary purpose of the shemittah of loans is to prevent eternal hounding by the lender. Therefore, there is no problem if the money is given willingly. The only kind of finagling discussed here is in a case where the borrower comes on his own after the end of shemittah. He wants to repay the loan, so it’s not hounding him to accept it. However, he isn’t allowed to repay the loan, since there is no loan. Instead, you should try to get him to phrase it as a gift. If you can’t, then you may not accept the money.