Tracking customer P.O./Balance in Dynamics GP

We have several large customers who cut POs for our services. They are not however for a fixed amount of service but instead are more like a "not to exceed". We then track invoices against those POs until the PO amount has been exausted and then let the customer know that a new PO is needed.

How could this be tracked in GP? The PO module seems to be for creating our own POs and not recieving them and I could find very little information on having customers "on account".