SPS 500 DISCOUNT

HI

Samsung sps 500

I am trying to program a function key to give a -10p discount for any takeaway item. I have set up a key as function 157 labeled it as discount and changed to halo to 10 and set it as an amount and a minus .

when you ring up and item then press the discount key - "discount" appears on the screen but no amount" and no discount is applied

when I change the minus to positive - it adds 10p or when I change from amount to % it deducts 10% but I cant get it to deduct 10p.

Any idea where Im going wrong?

Thanks