Making Modificaitons to Purchase Orders

Hello,

I was interested if there is something that we are missing on this and/or what other companies are doing in this situation.  We have purchase orders that are entered in the AP system that will be paid on multiple times (ex. lawn care).  Once you match as many payments to the PO as there is quantity entered you cannot match to the PO anymore even if the total is not reached.  Currently the only work a rounds we can find are to add a line to the PO let's say for a penny or to SQL the quantity field which