Reverse taxes charged in error

For 11.5.10 in Receivables, we have the following scenario:

-AR invoice created with tax (in this case it is a tax group that includes 2 tax lines - GST, and PST tax codes)

-invoice is in completed, posted to GL status

-customer provides a tax exemption certificate and pays invoice amount excluding PST tax amount

We would like to credit the tax charged for PST; and would like to handle the transaction in such a way that the tax reports do not include this invoice as a taxable amount for PST.

What are the proper transactions to correct this tax?  I have attempted both adjustments and credits and have not been able to specifically credit the PST tax only.