My credit card has a limit of $6000. If I want to book a $7500 vacation on Expedia, could I put a negative $2000 balance on my card, so when I pay for my trip on Expedia, it will get approved.?
It's possible that this might work, but it certainly isn't something that you should rely on. You should check with your credit card company to see what their policy on this is. Many do not allow it:
Bank of the West’s credit card agreement, for example, states: “If we accept a payment from you in excess of your outstanding balance, your available credit limit will not be increased by the amount of the overpayment nor will we be required to authorize transactions for an amount in excess of your credit limit.”
Some card issuers, such as Capital One, state they can reject any payment that creates a credit balance. “Any credit balance we allow will not be available until we confirm that your payment has cleared.”
If the $7500 is not all one transaction, you can make payments in between the transactions. You can also ask your credit card company for an increase in your credit limit. If you've had your card for a while and have a good payment history, there's a decent chance they'll agree.