Cancel Date on Contracts

Is there any way of processing a voucher for a contract that is past the cancel date other than changing the cancel date on the contract?  This can happen rather often where a contractor submits an invoice for services done prior to the cancel date, but produces the invoice after the cancel date (it may take several months for the contractor to get all the approvals needed to submit the invoice).  Right now we can not process any ivoices like this unless we change the cancel date (which we don't want to do for audit trail purposes) and we don't