If I repay a loan in a way that causes the lender to pay taxes. I’m I allowed to cover the tax bill or is that Ribbis?

One may not charge the borrower fees that accrued due to securing the loan, however, wiring costs, and legal fees for drafting the loan document, can be transferred.

Sources:

Bris Yehuda 9:2, C.M. 39:17; 106:1; Pischei Choshen, Halva’ah 2:41-42.