Here's the (hypothetical, of course) scenario: Reuven borrows $20 from Shimon. The easiest way for Reuven to pay back the money would be to send it through one of those new-fangled fancy electronic methods (for example, PayPal). Shimon is willing to accept payment in that manner.
The problem is that PayPal takes a percentage of the transfer ( 2.9% from the sender if it's a "transfer to a friend, 2.9% from the receiver if it's a "business transaction").
So the question boils down to this: Who is responsible to pay the processing fee when paying back a loan via a third party? For the sake of the question, let's assume that we're dealing with a default case where nothing was agreed upon in advance (although if this touches upon issues of Ribbis, it may not matter whether the parties agreed to it or not).