Voiding/cancelling a Check in a closed period

Hello,

I have a cheque that needs to be voided/accounting cancellation.  The problem is that it is in a closed period, and the entry is finalized.

The other problem is that this payment relates to a prepayment and needs to go back to the PO so that it can be done properly.

While this situation, with it being a prepayment, is unique, having to void a cheque outside of the month is was originally written in is not unique.  How does everyone deal with this? I can't leave the month open until everything clears (cheques are valid in Canada for up to 6 months). 

Thanks 

Erin