Sales Tax Issue with Dynamics RMS

Here in California, the sales tax recently increase from 8.25% to

9.25%. I made this appropriate changes to the sales tax settings in RMS. The issue is when I get a return for items purchased prior to the sales tax change, the customer is credited at the current sales tax rate of 9.25% rather than the 8.25% that they should be credited. I need to be able to adjust the sales tax percentage at the POS for these returns so that the sales tax amount is correct. Any suggestions?