It is possible in some cases that your purchases are overpaid, this is maybe caused by an admin error on the supplier side.  This means you cannot just mark the purchase as paid and you need to record the overpayment correctly.  You can either record this as money back into your account if the supplier returned it, or apply it to another purchase depending on what is arranged with your supplier.

Supplier returned the money

The first step in either option is to show the overpayment originally being applied.  To do this you will need to go the Purchase and then in the Payments section, you will need to apply the full overpayment by clicking Add Payment.  You may need to overwrite the prepopulated amount we have given.  Once this has been done, you need to press Save and then the purchase will be marked as overpaid.  Now you need to add a second payment to show the supplier paying you back the money and to get the purchase to be marked as paid and not overpaid.  To do this, again, go to the Payments section > Add Payment and this time you need to enter the date and the amount you received the amount back into the account as a negative payment amount > Add Payment.  The invoice will be marked as paid and the overpayment recorded successfully.

Applying the overpayment to another purchase

Just like the above, you first need to show the overpayment on the purchase first.  To do this you will need to go the Purchase and then in the Payment Details  section you will need to apply the full overpayment, you may need to overwrite the prepopulated amount we have given.  Once this has been done, you need to press Add Payment and then the purchase will be marked as overpaid.  Now you will need to go to the purchase which you want to apply this overpayment to.  You can do this by going to the Purchases and Stats section of the supplier account, then select the purchase.  Now in the Payment Details section you will see the option Apply Advance Payment, press this and then select the overpayment from the original purchase.  It will then apply the payment and also mark the original purchase as paid.  Both these methods will allow you to treat an overpayment in your account correctly.