House percentage and negative credit

Hi,
I am very close to having my website set up exactly as I want it, but there is one problem I can’t solve.

If a seller has 0 credits to his account and a buyer purchases his listing he will have a negative credit amount. When the work is marked as completed this resolves as credit is added to the account of the seller.

Is there any way to only change the Sellers credit after the completion of a job (when the seller marks the listing complete)?

Basically I don’t want a Seller to see a negative credit ever.

Best regards,
Tony