INT. A VET'S CLINIC - DAY
A man is stood in the room looking a little distraught. The vet is injecting something into a dog lay unconscious on the table.
VET
Would you like me to dispose of the body?
MAN
No, I want to take him home
VET
Ok
The vet goes into a back room.
Cut to the man, looking very sad. The vet hands him a dog-sized paper bag and the man leaves.
Erm, yes. It's like a Tommy Cooper joke but very morbid and sad. Poor doggie 
Only kidding, but seriously, it's an old joke mate, that's all I can say ^apart from that other crap I wrote.
I thought it was probably an old joke. A bit too obvious to have never been made before. Ah well
It is an old joke Chris.
As an alternative ending you could have the vet give the man a leash and cut to final scene of man dragging his dead dog towards the exit.
I wonder, if you order dog in a Korean restaurant, do they give you the leftovers in a doggy bag?