Go get the poor little guy. I find it cruel to take him back, you probably broke his little heart. When we got our chins one developed an eye infection 3 days after getting from a breeder, i contacted them and she offered my money back, i declined as the only reason i called was to find out a good vet in the area. I could never give an animal back unless its a life/death matter, a small stomach issue is nothing of the such. I know you had the best intentions at heart, but the constant moving isnt going to help. Think long and hard if you want the commitment that a chin needs before you bring him back home.
I hope the best for you and the little one. I would go get him and finish giving the medicine myself to make sure hes in a happy location with the most love possible, as we know petstores are not those.
P.S.- My first 2 chins were from petstores, I know where your coming from on this.