Regarding the answer concerning a person barrowing a half jar of horseradish and the returning a full jar. Why can’t we consider it a purchase instead of borrowing. The person is just purchasing the half a jar at a higher cost?
Ribis doesn’t depend (usually) on what you say, but on what you do. The Gemara records an opinion that wanted to permit interest by saying that he was “renting” out his money. However, this doesn’t help, because at the end of the day it’s a loan, and not a rental agreement. The same applies here. Nobody buys a whole jar for half a jar, and it’s clearly a case of borrowing some food, as is the way of neighbors. Therefore, one must be careful not to return more than one takes.