We opted to correct the books for "the year", being as we could not go back and add the transactions in the month. We recorded the sales in our current month. For sales tax purposes, we  reported the actual sales for the current month (although the sales Summary report stated overage, we made a note for the the prior months sales adjustment) and amended the prior months sales tax return. The result, the Quarterly totals and the annual totals are correct, only the 2 individual months were incorrect for the same amounts, which was the originally unrecorded sales.