@Dara155 I'd have them book through AirBnB again (request to book) and respond to them with a special offer that covers the credit they should have. I'm assuming that the amount they will need to pay should be fairly low to almost nothing. Since I think the lowest you can charge them is $10/night, you can offer to make up or refund anything over the original amount they paid. In my mind, it's worth the $10-25? dollars this might cost you.
That way, you have an official reservation through AirBnB, your guests are happy, and you are protected. I'd happily give up that amount in profit for a solution that accommodates this situation.