Does the man have ANY papers saying he has bought the dogs from you (or your BF)? If not, you can even report him as he stole your dog! Check with your vet if the microchip is on you name yet.
Does the man have your dog's vaccinations booklet? If yes, it wll be a bit harder to say he stole your dog.
Even if he lent your dog I guess you wrote some paper about it, didn't you?
But whatever happened and was written, if the dog goes well and is beloved, maybe it would really be better to leave him where he is and have another dogie.
YIS