I can honestly say that I really do hear where you are coming from on this, and I agree that it seems strange that we would ask you to come back to purchase an item at a different time just because that item is on sale.
Without me looking at the coupon you have directly it would be impossible to say for sure, but I would assume this coupon does state that it is not valid on sale items. I don’t believe this exclusion is intended to force people with coupons to come back at a later date.
It does seem strange that the store wouldn’t offer you any resolution to your concern, but I would like to personally look into this for you. I am sending you a private message to gather some additional information. To check your messages you should log into the forum, and then click on the letter icon in the upper right-hand corner of the page.