I would tell them that your rule is the dog comes back to you if the buyer can no longer care for the dog. Tell them you are sure you mentioned it when they took him. I would tell them since they neglected the training for so long he is going to need a lot of work training and maybe the person they are thinking of giving him to wont have the time to deal with training an adult dog
And, you fear that he will be passed from person to person. When coming back to you he will get the proper training and find a home that has time to dedicate to him.
Good Luck! I hope you get the little guy back!