Orders; Purchases

Paid a deposit to a supplier where the balance was due at a later date.

So I created an order and marked the deposit as paid (Which has since been reconciled). I was going to turn the order into a purchase when I needed to pay the remaining balance. 

The remaining balance has since been paid from a different system. so now I need to keep the deposit through the system but turn it into a purchase without affecting last months GST - as that has been reconcilled. 

I know it doesn't show up on my reports as it is just an order but I need to edit it to just show there is no longer a balance - how would I go about this?