Subway complaint: Overcharged and refused to refund the overcharge

Clerk accidently charged double for 2 sandwiches on my gift card. the manager,female brown hair, stated they cannot correct the error due to being a gift card. She was rude and started yelling at me. I told her I wanted to speak to her boss. She had taken my gift card and refused to give it back. She stated a manager was on the way. I asked how long before she gets here and she refused to give an answer. Subway charged me double and this manager yells at me? How could Subway possibly believe this was a good person to hire? The manager above her, Rhonda Turner, arrived about 10 minutes later. Turner also stated they could not reimburse me cash or give me another gift card with amount of overcharge. Turner stated the only thing she could do is give me a coupon for food for the amount of the overcharge and it would be good for only 24 hours!!! I find this despicable and dishonest. Subway overcharged me and gives me 24 hours to buy more food from them. Turner blatantly lied to me stating they could not reimburse me with cash or a gift card. In retrospect I should have called the Police and reported a theft, as this was clearly someone taking money from me. I would like to hear from you and hear your opinion of this incident. Is this a way of producing higher profits by overcharging customers and then forcing them to buy more product?