If you add 800 dollars as credit your client can use this balance to order new services and for renewals. I suspect that you don't want this to happen. Maybe you just want to register 800 dollars for statistics.
In this case you can simply INSERT INTO tblaccounts 800 dollars as "amountin". Don't forget to specify userid, paymentmethod, date etc. This way WHMCS calculates statistics (today, this month, this year, overall...) including this specific amount.
P.s. sadly I can't give you the exact SQL query. My gf is looking at me with this face... 😤