Freight GR/IR Clearing

The GR/IR for freight which is for staggered deliveries is posted thru T code MIRO against multiple GRNs but the invoice is for the entire liability amount. (ie: agst multiple line items of freight liability a single claim for Transportation cost is received which is accounted thru LIV. (say for example 3 line items of Rs 15 each got generated on 3 different dates & the consolidated claim for the three liabilities was received on the 5th day .The same was cleared under MIRO.
The system stores the 3 line liability items as GRIR (credits) with each line item of Rs 15 (total Rs 45 as credit) & fourth line item (single record) with Rs 45 as GRIR (debit). The clearing does not happen till the PO is completed/closed
With the above feature & wherein we have long term Po's (for a period of six / nine months or even for a year period & especially since the data is voluminous things become altogether more challenging.

Is there any way where by the clearing can be carried out with the PO being open.