Programming for NO sales tax?

I have a Sharp XE-A106 and I would like to program it so that I can accept a coupon or apply a discount that will reduce the taxable amount of the transaction. i.e. I would like to be able to ring up a $10 item, enter a coupon/discount in the amount of $1 and have the amount due equal $9 plus the tax on $9 instead of having it equal $9 plus the tax on $10. I've already programmed a DEPT key as a negative value and that works for reducing the amount due, but sales tax is still being calculated on the total price of all items. If this isn't something I can program in is there a manual way of getting the same result with this machine? Thank you.