Great job, OP!
Something similar happened to my sister. A man in line in front of her kept demanding he be given a discount for a coupon he did not have. The clerk was polite but did not back down. "Our policy is you have to turn in the coupon to receive the discount." The man became increasingly agitated and belligerent, finally declaring he was going to get the clerk fired because she was rude and unhelpful. He turned to my sister and said, "And you'll be my witness." My sister replied, "Feel free to list me as a witness but the only thing I'm going to report is what an incredible jerk you've been to this poor girl who is just doing her job." The man harumphed and stalked out of the business.