Just explain that you and he have a contract, your price was based on what was agreed in the contract, and you should be allowed to fulfill the original, mutual

Just explain that you and he have a contract, your price was based on what was agreed in the contract, and you should be allowed to fulfill the original, mutually agreed contract. Then, if the customer wants more, negotiate a new contract. If you don't do this, you risk moving into the verbal contract domain, and, as Sam Goldwyn famously said, verbal contracts aren't worth the paper they're written on.