The dog, named Midnight, is a one-year-old Yorkie-Bishon mix. He was taken to the Pets Plus store in Delran for grooming.
After his grooming, store workers say Midnight's owners weren't able to come to the store right away, so an employee took the dog outside to relieve himself.
While outside, Midnight slipped out of his collar and ran away.
Workers say Midnight ran from the front of Pets Plus, across Route 130 and into the Walgreens parking lot at the intersection of Route 130 and Fairview.
He then slipped into a dark area and workers lost sight of him.
Pets Plus says it is offering the $500 reward for Midnight's safe return, no questions asked.
Anyone with information is asked to call Pets Plus of Delran at 856-764-4414.