zen cart paypal deposit payment module where customer can make a deposit on the order total via paypal.
e.g. $100 deposit; and rest is payable on delivery
admin can set manually the deposit $amount and paypal address
maybe clone the paypal IPN module and edit the variable input $amount to be user specified (via code); so always sends request of $100 to paypal no matter what zencart tells it to send;
customer will return to zen cart store after payment and it says the "Deposit Payment option" has been paid.