You can say "if you are dressed like a clown today we will give you a 10% discount". Point being that there is nothing to prevent you from giving someone a discount as long as it doesn't break any laws and as long as you are offering it to everyone that falls into the same category (or the discount has been negotiated).
Nothing but your merchant agreement. I worked a register in a small store when I was 16, and in a slow moment I ended up reading some of the stuff VISA sent along with their booklet of invalid credit card numbers. (Yes, this was a while ago.)
In in the fine print it was clear that offering your customers a cash discount was grounds for immediately canceling your ability to take credit cards. If I recall rightly, I noticed because the store I was at did indeed offer a discount. In practice, I'm sure the store was small enough that VISA didn't care. But for larger operations I'm sure it was a serious threat.
I believe they were eventually sued and lost, but since I rarely see discounts like that, I imagine they have some sort of not-quite-illegal trick, like offering merchants advertising credits based on charged card volume.
You can say "if you are dressed like a clown today we will give you a 10% discount". Point being that there is nothing to prevent you from giving someone a discount as long as it doesn't break any laws and as long as you are offering it to everyone that falls into the same category (or the discount has been negotiated).