Pelin, In English, we would say, "I will pay you back, with interest" or "I will pay you back, plus interest." The first sentence is more formal. As casual shorthand, at least Americans (can't speak for our British counterparts) like to say "plus this" or "plus that" when talking about adding something like in this example or in this one: "I went to the store and I got a bag, plus a few clothes." Let me know if that answers it for you. Best, Francesco

The sentence should read "I will pay you back, plus interest." Or, "I will pay you back, with interest". It is not necessary to have both "with" and "plus"

"I'll pay you back with interest" sounds better.