>How can i ensure that if for any reason she can no longer keep him he returns to me?
You would hope that money is not an issue but it does seem that payment for the dog was a factor in the other case. The breeder had stated that they wanted their puppy back with no payment/refund, and the owner wanted to recoup her losses, or at least not allow the breeder to sell her dog twice.
Perhaps you should be careful to say that you are willing to have your puppy/dog back, if their circumstances change, and that you will help find a new home BUT do not mention money.
Presumably most breeders look at this on a case by case basis, age of dog etc. If the breeder was able to find a new home, and resell a puppy/dog then they would be willing to pay some of that money, less costs, to the original owner - if only to bring things to a close?