None of the existing payment gateways will allow you to store a credit card number.  But, as long as you and your client understand the security risks of doing this, there are a number of ways you could achieve it, though all will require you writing a new payment gateway to collect the card details.

My advice would be to write a completely new gateway, perhaps using the Authorize.NET gateway as a template.  In terms of storing the card details I would suggest:

1) Do not store the entire credit card number in one place (the database or otherwise), especially if the database is on a shared hosting environment.  Consider splitting it into two parts storing the first part in the DB and the second to be (optionally encrypted and) emailed to your store administrator.  If you really cannot split the card number then encryption is required as an absolute minimum.

2) This is a no brainer, but make sure your client is using SSL for the page hosting the credit card details collection form.

3) Beware of any data protection issues that might apply as well.  Some payment gateways require your systems to undergo audits for security before they will allow you to take payments in this way (this is the case with Protx for example).

Having just created a custom shopping interface for the company I work for I have to second the Idea of using Authorize.Net, however it is even better for what you would like then previously mentioned.

One of the payment processing methods is called "Authorization Only." What that does is send the request in and verify the funds are available. The merchant then has 30 days to send a follow-on request of the type "Prior Authorization and Capture." The transaction is not sent for settlement until this is done. I have not played with this method myself so I am unclear if the credit card number is required for the second half of the transaction or not, but I bet Authorize.Net support would have the answer for you. I would assume they do not need the credit card a second time as that would force merchants to store credit card information, but again - I do not know for sure. This method of processing is specifically identified in their API as for situations where a merchant needs to check stock availability or review orders before shipment of goods.

There is one person who is implementing this on a different platform than DNN and used C# to write a current implementation. The link to his site is http://www.developingfor.net/miscellaneous/authorizenet-c-code.html. I have actually written a DNN implementation for it in VB and will be posting my code examples for initial implementation (mirroring his but on the DNN platform) this weekend on my blog at http://www.myhrfamily.net. Sometime in the next couple weeks after that I'll be posting my expanded implementation as I have built based on his to be a fuller implementation.

That said, the built-in PayPal processor may have some kind of delayed Capture type of transaction available, and other credit card processing products may as well. I just do not have experience with them and cannot steer you in the right direction.