Can my employer take discounts they give customers out of my commission?

I'm dog groomer in Massachusetts and I work for 50% commission. My employer offers various discounts for customers such as senior, military, rebooking discounts, breed day discounts, etc. All of these discounts come off of the price before it goes to the register, meaning a 100 dollar service with a 10% senior discount becomes 90 dollars before it gets to the register, and I get 45 instead of 50 in my check. I take five dogs a day every day with or without discounts. The only difference is that when the services are discounted, I lose money. They have said it's justified since the salon overall is making more money, but individually we are not doing any better because to do so, we would need to take more dogs daily anyway. Is this legal??