First of all, the dog survived. Let's get that out of the way.
This one comes to us from the ever popular subreddit, Am I the A**hole
. And right off the bat I can tell you with full confidence that the OP is not the a**hole.
If you are a dog/cat/pet owner of any kind. That pet is your responsibility. Hands-down, no questions asked. So when the OP's neighbor's dog accidentally got out, walked over to the OP's property and started eating covered food that was left out. That is not on the OP.
That's squarely on the dog
owner. Is it unfortunate that the dog happened to get into chocolate and grapes (two things very bad for dogs)? Absolutely. Is it the OP's fault? Absolutely not.
To the OP's credit, they offered to pay a quarter of the vet bills. Well that apparently wasn't good enough for the entitled dog owner...