Like others, I would wait a couple of months - I've had things come up as much as 4-5 months after the original charge (and a phone call from somewhere asking me to pay a bill, because they realized a charge from months ago never got processed, and my credit card had expired!)

If you need to give the company the money back, the way I have always done it (things like expensing a flight bought ahead of time, then ended up cancelling and getting a refund to the card a couple of months later) is to put the other expenses on the form as normal and then enter -700 as a line item with explanation of 'reimbursement for [reference original expense form and attach a copy] as amount never charged to credit card'. Not complex from the account department's perspective as long as they can tie it up to the original expense (the copy will help them).