If the cheque has been issued it is easy to void. Go to the Payments module and get the vendor's record. Near the top of the screen there is a scroll icon, click on this and it will open all of the paid and unpaid invoices from this vendor. Find the invoice which was paid by this cheque and enter the same cheque number with a suffix of REV to indicate 'reversed'. Use the same date and enter the amount as a negative number. When posted the cheque entry will reverse the amount and leave the invoice unpaid.

If the manual cheque has not be entered in Sage there is no easy way to make an entry for a VOID cheque as it relates to nothing in Sage.