Simple Mathematics, The Story

Okay, so you've had time to ponder what you should receive, right? Simple math says you should be returned "a" and "b". Simple. Well, that's not what happened.
When we returned a $40 item to a store ("c), the ladies told us that our $5 coupon ("a") was going to "roll over" and "carry over" to the exchange. We'd lose the coupon if we just returned it, but it was going to carry over since we were getting something else. However, when they rung us up, that was not the case. They only gave us back $35 ("b"). We tried to explain this and got in quite the discussion about what they were saying versus what they were doing. If they would have said that the coupon was no longer valid and that was store policy, so be it. However, they continued to say that it would roll over to the next product, yet couldn't figure out that this meant they would then have to give us a $5 discount on the new product. In the end, we tried to leave the store but they wouldn't let us. The manager did give us the $5 off. The cashier then stated that they had then given us $10 off the product (simple math, remember?). We couldn't believe how silly this had become and it was a simple matter of knowing what "roll over" and "carry over" actually means. To us, it meant that the $5 coupon still existed. To them, it only meant that we'd get credit for the $35 we actually gave them and the "roll over" really meant that it was credited TO the returned item, not the new one.