I want to know what is the best way to manage the below activity

I make an invoice to the client. Lets say of 10k. After that I add Fixed cost Project under that clients name in time sheet. I set the budget hours as 10k / 500 = 20 hours. Then when my staff works they add hours under that project. All this works fine.

But later if I am raising a new invoice to that same client, the initial 10k comes up in unbilled hours. I do not want to bill that client. for 10k as the invoice has already been made.

What setting do you suggest I make for the same.