Refunding a deposit payment

A customer has paid a deposit of £200.00 by debit card which is the only transaction on his account. He has cancelled his order and we have refunded his debit card by £200.00. I have gone to Money, Manage money, Refund and entered the refunded amount, but by doing this it allocates a refund to debit card but raised an invoice to cover the refund. When you view the customers account it still shows that his account is £200.00 in credit. How can you allocate just the payment side of the refund. I have tried doing a negative amount by going to sales and money in but it doesnt allow you to enter a -£200.00