They cannot change the terms of the agreement (whether a written or unwritten/oral agreement), so if there was no requirement for payment in full prior to delivery, they cannot require it now. That said, if they refuse to provide the product without payment, the only way to force them to do so would be to sue them and get a court order for its release/delivery. You need to decide whether it is better to sue now, or else pay and then sue (e.g. for breach of contract: not providing what they agreed to provide, which is a refurbished and working machine) later if need be.