Opossums have a very thick skin and take a pretty hard bite and never get their skin pierced. Opossums will play dead. That's their defense mechanism. Your dog probably gave up think the opossum WAS dead. When critters fight back, it gets dogs even more excited and the killer instinct gets even more vicious. But one they are "dead" they quit. The excitement is over. Once the problem went away (your dog), the opossum simply got up and moved on down the highway of life!
However, if an opossum ever gets cornered and does decide to fight, they are an extremely vicious animal. Their teeth are pure evil! Be glad your dog gave up.
Also, don't leave food outside.