Hi Jennifer,
I don't think without making any changes in the clearing rule, you will be able to cater your requirement.The system has to be feed by the logic to clear the P2P items first ahead of the normal receivables in the customer account.Check SAP Note 1376818 as well.
Two possible alternatives which I could think off-
(a) The P2P items against the customer account can be put on direct debit. Standard SAP has this functionality.This will eliminate the clearing issue.
(b) Lower the fulfillment criteria to 80%-85% based on the Business requirement. Though not a foolproof solution, but it will take care the most of the P2P scenarios.
Hope it helps...
Thanks,
Amlan