If a mistake has been made and receipt should not have been issued. You can easily void a receipt in fundraisingManager.
From the gift/donation record you can edit the status of a receipt to Void.
If the receipt was lost and you need to generate a replacement receipt. You can click "Create Replacement Receipt" button in the top left of the receipt. A new receipt with a new serial number will be generated and even emailed to the donor if necessary.