Trying to finish our month end. In Job cost a billing and payment batch has an invoice dated 2/15/2006 for $500.00

The invoice number in the job cost batch is an invoice created in invoices - dated 5/25/2005 with 10% retainage of the total invoice ($5,000.00)being held, leaving the invoice with a net total of $4500.00. The date I entered that the retainage is due is 2/15/2006. I'm very confused. I never created a "retainage invoice" for this invoice. Why did the 500 carry through to job cost, and not the 4500.00? In invoices, when I tried to create the retainage invoice for this, it showed that there was no retainage for the invoice. that the amount of the invoice is 500. Checking the original invoice in history, all of the correct (as I understand it) information is there. retainage: 500: Net total of invoice: 5000.00

Thanks for any help!!
Karen