
We had a pup back here at 9 months. I trusted that the owners wanted the best for their dog, and didn't mention money to them either. I didn't have any wording in my contract regarding monies returned, although like others, would have paid if needed.
As it turned out he was a lovely boy once he'd settled in and learned some manners. He was eager to learn & such a handsome chap I would probably have found a way to keep him (he was show quality & would probably have done well if shown) - unfortunately he was already been castrated and wouldn't stop badgering my male (also castrated) to wrestle, hanging off him constantly (unfortunately my boy is very easy going and never once told him off - we just wanted him to be "told" as he learned pretty quickly after his mother set him right after he tried mounting her!). He had settled in so well here and it was an incredibly difficult decision but in the end, after advertising locally to no avail, I contacted our breed rescue, who had the most perfect home imaginable - country home, breed experienced people etc, so let him go.
I must admit, it was a very difficult decision, but feedback was that he strolled into his new home, immediately making himself at home!! The biggest shame for me is that I don't get any feedback as he went through rescue. I think about him all the time :(.
To the OP, I would just arrange a date/time for them to bring him back (if that's what they want). I wouldn't even mention the money side, as IMO anyone who signs a contract requiring them to return the dog if they can't keep it, relinquish the automatic right to a refund - especially if they've had the dog for a few months. After all, the breeder then has the awesome responsibility of retraining the pup/dog, assessing them, and placing them in a suitable home. One of the pitfalls of dog breeding, and I hope it never happens again as I found it really difficult not to be emotionally involved.
Good luck, hope it goes well.