As you said, you are the guarantor of the loan, and you are afraid of taking joint liability. You tear off the borrower's signature and guarantor's signature on the IOU for your safekeeping, and the original has been copied or photographed. In other words, the existing original is incomplete, and you can't see who the borrower and guarantor are. This IOU lacks the necessary elements and has become a piece of waste paper. Even if there is a real loan relationship between the two parties, it can only be a verbal agreement between the two parties. The original IOU is invalid. Either re-sign or re-write. If you don't want to guarantee, you don't have to sign it. You can go to court for a lawsuit. You can use a copy when accepting a case, but you must see the original at the trial. Your behavior belongs to guarantee.