Auto close PO once paid

Hi There,

  We are using PeopleSoft 9.1: We usually buy cable using a PO. Majority of the time we do not know what is the lenght of the cable so we make the PO +- 5% as the tolerance amount and quatity.  Our issue is when we receipt for the PO and it's less that the PO price, the PO stay open until someone closes it manually.  E.g. PO is for $100 for 100ft and we only receipted $99 for 99ft. Is there a way we can have the remaining PO auto close? We want it close because it is just