It depends on the system whether you can send the tip later but the processor will have it for a while. If it's been more than a few days, I would let it go, don't bother but within a few days, most customers would expect to see the tip added at some point and wouldn't be surprised.
Having the card number in your system is okay as long as it is secured properly. If your system does tokens, tokens are usually good for about 6 months, depending on the processor.