Can one pay more on an Installment Agreement than the negotiated amount?

You are on the right track wanting to pay off the IRS sooner, for by doing so you pay off the Agreement early and will save interest.  You do not have to renegotiate a new Installlment Agreement, although you can if you wish.  Instead, you can add whatever amount you want to your monthly payment, e.g. you can voluntarily pay $500 instead of $200 per month.  This additional amount can change each month.  Please keep in mind; however, that you will need to pay at least $200 each month.  In other words, last month’s surplus payment does not create a credit for this month’s payment.