He will (or should) eventually return to the seller, at which point you will need to pay 500 gold for him again if you want him to continue travelling with you. If/when that happens, don't let him die because you can't buy another one if he does.
The problem is that he was a stray dog who i found while adventuring... so i didn't buy him, he just agreed to follow me and now he is gone. I didn't get any message about him at all...
I had a stray dog. He was cute. He all of a sudden refused to continue in the direction I was heading (plain path) and stopped dead in his tracks.
I left him and did the nearby cave I was heading to, but when I came back, he was gone. This was 15 levels ago, and I don't remember seeing a
message of him leaving. Maybe I just missed it. I never tried to buy a dog though. No one ever offered to sell me one. Don't think I want another
one anyways. he was cute, a little helpful, but ALWAYS in my way...always.
I think that you are in the same situation like me, you wouldn't be able to buy a new dog if i'm right.