R12 Correct way to create a sales tax only credit memo

1) R12 Receivables

2) Created an invoice via AutoInvoice based on an OM shipment which included tax.

3) e.g. Invoice total $1,250.00 - Products = $1,200, Sales Taxes = $50.00 for State=CA, County=Santa Clara, City=Milpitas

4)  Ran Sales & Use Tax report and filed sales taxes for CA

5)  Later determined that the customer on the invoice should have been exempt

6)  Need to create a credit memo for the sales taxes only - $50 AND have the effect of the credit memo correctly shown on the next month's Sales & Use Tax report

7)  What are the necessary steps for this?