If you make a payment to your card today, then as soon as that payment posts, that amount will be returned to your available credit.



If you don't have well-established credit and have only had the card a few months, they probably won't honor a request to increase. Chances are better if you've had it at least six months and always paid on time, and even better if you've had it a year. I don't know if the UK has the same concept of "hard" and "soft" credit pulls like we do here, but usually a request to increase a credit line is either done solely based on the card-issuing bank's experience with you (i.e. no credit check is performed) or they do a "soft pull," which does not show up as an actual credit inquiry on your credit report. Some, though, do a "hard pull," which does show up on your credit report. If you call and ask for a credit limit increase, be sure to confirm whether an inquiry will be performed on your credit and what type of inquiry it will be. Even if it's a hard inquiry, though, it usually (at least here) has a very minimal and relatively short effect on your credit score. That said, it's probably pointless for them to do a hard inquiry on you because your credit has likely changed little from when you first got the card a short while ago.

I've actually never paid attention to how they show up on Hertz receipts and/or whether Hertz's POS system is even capable of it, but it is definitely possible for a merchant to perform what is known as an "incremental authorization," which changes the amount of the existing authorization rather than getting an entirely new, separate authorization, as long as it's a Visa card. This also has the benefit of allowing the merchant to settle the total amount in a single transaction, whereas multiple authorizations have to be settled separately--for example, if the merchant obtains three separate $100 authorizations and your final bill is $300, they have to settle it as three separate $100 charges.

Of course, systems can be a little dumb, and so there are a couple ways that they often program the systems poorly:

-at the final close, two of the three authorizations are entirely disregarded and the $300 charge is posted against one of the $100 authorizations (this is a violation of the merchant agreement, and there is a small penalty assessed for settling a charge for more than was authorized)
-at the final close, the previous authorizations are entirely disregarded and a single, new $300 authorization is placed and the charge is posted against that authorization (technically a violation of the merchant agreement but not really possible to easily detect or enforce)
-at the final close, two of the three previous separate authorizations are discarded and the system performs an incremental authorization of $300 on one of them and posts the charge against that one (only possible with Visa, as MC/AX/DS don't support incremental authorizations, and better than the first two situations but still a slight violation, because merchants shouldn't make authorizations they don't intend to use)

Visa also supports "authorization reversals," which are basically reverse-incremental authorizations. With MC/AX/DS, a merchant is free to settle an amount for less than the authorized amount without a penalty. With VI, they can't--the transaction must always be within (IIRC) 15% of the authorized amount, above or below. So if the merchant authorizes $500 but only needs to settle $100, they need to perform an authorization reversal of $400 before settling the charge. Incidentally, with Visa, it's also possible to effectively void an authorization by doing a full reversal down to $1, thus freeing up the customer's credit (how quickly the customer's bank responds to this, if at all, is wholly dependent on the card-issuing bank, though). At least as of 2012, when I was last intimately involved in the world of credit card merchant processes, MI/AX/DS did not support reversals, but a quicker way to free up a customer's credit line is to post a $1 charge against the larger authorization followed by a $1 refund, because when the charge posts to the customer's card in a couple of days, the authorization is considered used and goes away.

Anyway, in your case, I suspect you used a Visa and that Hertz actually is performing an incremental authorization on your Visa, hence the authorization number remaining the same, and then your final charges were settled against that authorization. That's actually (mostly) the correct way for them to program the system (though they should have only authorized $78 minus $39, not a full $78). If it wasn't a Visa, then I'm out of explanations. That said, I think I've heard some rumblings that MC and maybe AX have been talking about implementing incremental authorizations/reversals, so it's possible they've now mirrored VI's practices.

Edit: it does seem that MC now supports reversals: http://blog.directpay.online/masterc...tion-reversals, https://www.mastercard.us/content/da...sing-rules.pdf. And here's the similar Visa guide, which talks about incremental authorizations and reversals as they apply to the car rental industry: https://usa.visa.com/dam/VCOM/downlo...l-industry.pdf. It also includes this interesting and helpful-to-consumers line: "Car rental agencies may not use a Visa product to process delayed or amended charges related to car loss, and/or theft, damages, unless the car rental company reserves express agreement to the specific charges from the cardholder."