Interest on Indirect Debt

If someone “indirectly” owes me money—not that I lent him money directly, but rather I paid off debts that included amounts on his behalf and on behalf of others, and since he owes them as well, I am now receiving the money from him (based on the principle of shi’abuda d’Rabbi Natan)—and the total comes out to 43.98 shekels.If he pays me 44 shekels, is that allowed? Or is there a problem of interest (ribbit)? Should he therefore pay a different amount—and if so, how much?

Shalom u'vracha 

There is no issue of interest in this