> They have said they will want a full refund and for me to sello him on if they do have to rehome him
Marianne and Brainless have given great advice about the value of the pup being less once it's older, and about fees for food and board etc.
Did you have a sales agreement in place when you sold the pups? If not it might be worth sitting down and composing an agreement about what you are willing to offer and what you expect from them. Putting cost to one side for the moment how long do you want to leave it before taking the pup back?
The puppy belongs to them so it's their choice about how long they wait but, if you think there is a very high likelihood of him being returned, then it may be worth incentivising them to do it sooner rather than later. You could check wording with some breeders on here otherwise you could adopt some kind of scale based on how rehomable the pup will be. For example:
For pup up to age xxx - full refund of purchase price - payable on return
For pup up to age xxx - refund of purchase price, less costs - payable half now and the balance on rehoming
For pup up to age xxx - payment of rehoming/sales price, less costs - payable after rehoming
This is based on the ease of rehoming - the younger it is the faster you should be able to find a new home. If the pup is much older then it will take longer and cost you more whilst you wait.
The difficulty with this option is that they might decide to rehome themselves to save money, which presumably you'd rather they didn't. If in doubt pay the price, get the pup back as soon as you can and chalk it up to experience. It's sad for the pup to be messed around but if the owners have the pups best interests at heart they won't argue about prices and won't wait too long before deciding - they'll want to do the right thing for the dog and daughter. Good luck