give me a while to think about it. You can't be guilty of theft - you have a defence if you have a written agreement and honestly believed the goods were yours to take. However, this type of "loan" may well fall foul of Consumer Credit agreements/usary type loans.
I think you need to put the exact terms of the agreement (not a synopsis) and the amount, and the date and the total payments made if any and I'll think about it.