PO with 1 line and qty=1, invoices multiple times a year

We have the case where we create POs for maintenance for equipment that is invoiced each quarter.  Since the PO has a single line and a quantity of 1 each quarter when the invoice comes in the invoice fails in the interface because of the qty/amount difference.  See Example

PO Created for 1 line quantity =1 amount = $5520

Invoices received

Jan-2013 for $276

Feb-2013 for $276 and so on for each month of the maintance agreement

How can we process these invoices wiithout human intervention..have the invoices applied to the PO each month and pull down from the $5520 and not be placed on hold??