create a product (service type) and call it "advance payment from customer); available for the POS.
Create a specific payment method for the POS and a specific accounting journal to record these payments (debit bank account and credit customer account ).
At the time of payment a cash ticket will be printed.
when the customer confirm his order for product XYZ you will be able to deduct the payment in advance (adding the prepayment with a Minus sign on quantity for the amount already received in advance); it should make the trick; waiting for a proper and more integrated solution from Odoo. (at the time I am wrinting this may have been done for V10)