Let's experiment. $3.97 * .1 = .39 (Okay, so it's actually .397 but I would allow them to truncate the fractions of cents) Now, $3.97 - $0.39 = $3.58. The didn't need a button, just a pencil and paper at the most. If they rounded the .397 up to .40 they should have been able to do it in their heads. But no, they needed the button that the proprietor had apparently disabled. Now, why would the proprietor disable the discount button and then advertise the discount?
Times sure have changed. Back in the early 70s I worked at McDonald's. I had many jobs during my tenure there at the "Golden Arches" on North Main St. in Elkhart IN. One of them was working the counter and taking orders. Back then we had a pad of paper and pencil to take the order and add up the cost. We then entered the total into the cash register, took the money and made change. The cash register did not tell us what the change should be, it only kept track of how much was added to the drawer. Times sure have changed.

If I were a mean sort of fellow, I'd go back to the Subway order the sandwich and then when they told me "We don't have a button for that." when I hand them the coupon, I'd tell them to keep the sandwich then.