Sales Tax

I invoiced a customer for the total amount of a job, and the job was taxable.  The customer had a credit on their account so I applied the credit to the invoice.  Everything looks correct on the invoice.  It shows the total amount, the sales tax amount (on the total) the applied credit and the balance due.  Today when I went to run my sales tax report my sales tax liability and taxable sales are off the amount of the credit.  Credit was $150.15 and my sales are off $140.00 and sales tax $10.15.  How else could I have done this to make sure my sales tax liability report is correct?  The credit was from a duplicate payment they paid so it was not for discounted items they said just keep the check and apply it to their next invoice.