I am buying two gym machines from a commercial gym that went out of business next week. The problem is he can't hold the two machines for me and I can't pick them up until next week so I'd like to pay him now so he can hold the two machines for me. Do I need to get anything more than a receipt of payment from him in order to not get screwed over? Is there any document I can create and get him to sign it? Please and Thanks!