If you have a receipt allocated to an invoice then you cannot reverse the receipt transaction until the invoice has been unassigned.
To do this, find the receipt transaction in History. If you're unsure of the transaction number, you can find it on the Tenant Invoice Report.
When you find the transaction, click on it.
Next to the Invoice Number section on the left-hand side, click on the link "Clear Assignment". Once this is clicked, the Invoice will now be unassigned and you will be able to reverse the receipt transaction.
Now you can 'save and close'. Click on the drop-down choose reverse transaction.