i recently rehomed my dog...?

they took on the responsibility for the dog. he is their responsibility unless you signed a health guarantee. They should have taken him for a wellness check as soon as they got him. You did nothing wrong, and I'm thinking this could just be an enlarged lymph node that will resolve itself, or an insect bite.

I'm with Moondog here. Something is off, and if this was my dog, I'd be in my car and bringing him back. Certainly I'd want to see this lump for myself, and have my own vet check it out. It's your decision but for the sake of the dog, I'd do whatever it takes.

Another possibility would be to have their vet talk to your vet about what he's found, so you know more about what's going on!! Vets will speak to each other, although rarely to somebody who isn't their client. I assume their vet has seen this mass?

ps I don't want to alarm you but given the area of this lump, it could be lymphs which although they do come up when fighting infection, this could also be something more sinister. If it is a nasty, I'd certainly want to be the one who has the dog treated - especially if this family has only recently taken on your dog. Timing could be better, for sure.

It seems the new owners have changed their minds about keeping your dog.

You need to collect the dog and find another owner for your dog.

Don't mess around with these people. A reasonable person would take the dog to their own vet. They know you wont' make 3 x 5 hour trips and are just making excuses.

This is crazy.

I would bite the bullet... go and get him... refund their money and sort out another home.

Phone Great Dane Rescue and try and go through them, once you've found out what the lump is.

it is now their responsibility to take the dog to the vet. Once you left the dog with them , it is now theirs not yours. Only your responsibility if you gave them a health guarantee.

Does not mean you don't love the dog if you make them take care of the dog now that it is not yours.