Depending on the card issuer when capturing an authorization or hold on a credit card you may be able to adjust the amount prior to capturing it as a sale. 

Card Brands

Visa/Mastercard/Discover

With these three brands you can take an authorization of $500 and then partially reverse the authorization to an amount less than $500 before capturing the newly lowered amount.

American Express

With American Express if you take an authorization for $500 you must capture the entire $500 and then issue a separate refund to the card. You cannot change the authorization. Your alternative option is the reverse/refund the authorization and instead take a new sale for the proper amount.