How to give customer a discount?

[digger vermont]

Hello All,

So I post an invoice for say, $104.00.  The customer gets their wallet
out and I say, "Aww, just give me a hundred."  I process the payment of
a hundred and now the customer has a balance due of $4.00.  Seems like I
should be able to now process a payment for the $4.00 and charge it to
Expenses:Discounts, but that doesn't work.

What is the Gnucash way of dealing with that?  The only way I can think
of is to make another invoice just for the discount.

Thanks for ant help,

digger