Darling, at the end of the day, did the Store Director let you use the credit on a B/K? There is the answer. You are not alone. I have heard of this happening to others I know personally and others on the forum.
That is the standing policy. I don't understand why you would keep a credit if you are buying more things but that is your right. Giving you conflicting information is not right either but many times SAs don't know about these non-written policies until faced with dealing with one. I find that we know more as members of TPF than they do. I really don't understand a lot of things Hermes does to the customers. Some SM are much more customer friendly than others. Some SAs know a lot and some know shockingly little...
I have been subjected to horrifying things by a SM that no longer works for Hermes. If you are willing to challenge the SM, maybe you will win. They know they hold the cards because you are afraid to ruin your relationship. Been there. It's terrible. I challenged, I won, I no longer shop at that boutique.... even after he was removed.