Using Paypal Balance & Debit Card for Single Purchase

Description

I am based in the UK, and I have a balance in my PayPal account in USD, because I was paid by a client in the USA. I have no other currencies in my PayPal account (my GBP balance is 0). I am trying to complete a transaction online for a product that is being sold in USD. The balance of my PayPal account is around $200 less than the product I want to purchase. Rather than be charged a currency conversion fee by my bank, I would like to use the balance of my PayPal account to contribute towards the purchase of the product, and then pay the remainder via my debit card. However the only option I see when trying to complete the purchase is to use either my debit card or credit card on my PayPal account – there is no option to use my PayPal balance. How do I use my balance first, and then have the small remaining amount charged to my debit card? Also is it cheaper (from a currency conversion position) to use a debit card (which is based on GBP) or a credit card?