You do have a receipt, but it does not contain any serial numbers and pretty sure their arent any printed on the sub itself. but if you have receipts for all of the other items you bought, you can prove to the cops that your theory of fronting him money is true. also you can supply a bank statement of deposits showing that you DID receive some cash from him. other than that you have no other legal binding documents. and also you might be just a little bit out of reach for the small claims court, which i believe is $500. just annoy the shit out of him for the money, or just......take whats your. PAY UP SUCKAS!