Paying Up a Debt of his Fellow

The Gemora (Kesuvos 108a) rules regarding one who is forbidden by a vow to derive benefit from his fellow; the fellow is prohibited to repay his debt (in a regular case).

Reb Elchonon Wasserman discusses the rationale behind this ruling. Is it prohibited because the one who pronounced the vow is directly benefiting from the money that is being used to repay his debt? Or, perhaps it is because the fellow is causing the lender to forgive the borrower for his debt (once he has the money from elsewhere), and it emerges that he is indirectly receiving pleasure from the fellow?

This question is actually dependent upon a different question: Can someone pay up the debt of his fellow and cause that the debt has been paid? Or, perhaps only the borrower can repay a debt; his friend may give money to the lender with the stipulation being that the lender will forgive the borrower from the debt which he owes?