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I am sorry to hear about this situation. Actually, they are correct, I am afraid. Allow me to explain why.
You signed a contract to be given credit. This means that they would cover purchases for you, not that they would advance you money. Some credit card contracts allow advancing money in the form of credit, but not all. Unless your contract specifically stated that money can be advanced in the form of credit, then there is no mandate for the company to advance you money up the credit limit (although they can/do cover purchases up to the credit limit).
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