Does anyone know how to include retainage on an invoice or for a customre?
Retainage is your Other Current Asset, if you are an accrual basis entity. Otherwise, it is a reduction of Income now, for being income later. Make a Service or Other Charge item for this. linked to Other Current Asset or Income (whatever meets your accounting requirement). Name them both Customer Retainage.
Use the Retainage item with a Negative on the invoice or sales receipt. This reduces the amount the customer owes, as an amount you will collect from them later.
Later, you use that on an invoice or sales receipt to show it is now Earned by you and that it is yours to keep.