Hi everyone!

I have a trouble with the process of making down payments for customers. I'm trying to do the next steps:

1. Register a down payment to a customer. [Ok, no problem]

2. I bill that down payment (no sale order yet). [Everything fine] 

3. Days after, I make a new sale order for that customer. [ok]

4. I bill that sale order. [ok]

5. I need to apply the previous down payment (step 1 and 2) to that new invoice [trouble]


In the traditional case, the final customer invoice has a zero total amount because of the down payment, but i don't know how to do this in odoo or if there is a different way to have this result. 


I'll appreciate if some one can give me a clue

Best regards to all!