This is really a "contract" question. As you say, they provided an "Agreement" between you and them saying you agreed to their terms allowing them to take the funds from your account. Once a contract is made, you CANNOT, on your own change the terms.
Imagine, (for instance) that you were going to buy a bike from your neighbor, and paid him the asking price on the condition that he deliver the bike to you tomorrow. Then tomorrow he says he has decided (on his own) that he still needs the bike, and is not going to give you the bike for 30 days. This would be a "breach" of your agreement and I think you would not be happy. If you took your neighbor to court I believe you would win.
I also believe that in this situation THEY would win in a fight. That said, the best advice I can give is STAY AWAY FROM THE PAYDAY LOANS!!
Wish I had better news, but that is the way I see it.
Do you have money in your bank account? Is it a direct deposit account? What is being deposited, wages or social security? I suggest that you change banks. You will still owe the payday lender, but you will be receiving your income.