how much do i legally have to pay per month in the settlement of a judgement against me?

I have offered $50 per month and the opposing insurance company agreed but sent a document saying they can change the amount paid at any time up to $500 per month. That seems rather drastic. I do not want to sign it and my lawyer is encouraging me to accept. Legally, what should I do? I was at fault in accident and was found responsible for the difference between my insurance coverage (approx. $10,000) and judgement (approx. $15,000). I would just like to get it hashed out, but am concerned that they will keep raising the payment until I can no longer afford to keep up with it.