The agreement you entered into is a contract. A contract may not be changed after the fact unless all parties (that is, you and him both) agree to change it. As long as you have been honoring your obligations, he is not entitled to anything else or any faster payment than the agreement says--your obligation is to do and pay what you contracted to do or pay, when you are supposed to do or pay it, and no more. He cannot make you pay the money faster than the agreement says.