Project Accounting Retainer

Hi,

We entered a retainer invoice and posted the invoice.  We now realize it was a mistake and need to back out the retainer. 

We are entering the retainer fee as a negative amount.  When I click OK in the billable fees window, I get a message saying "Returned Retainer amount cannot be greater than the project On Account amount."

The On Account amount is zero.  Why am I getting this message and how can I fix it?

Thanks,

Steve