So sorry to hear. Everything that needs to be said, was said by others. Just feel good knowing you gave him a comfortable, warm, clean final home. Like a brief retirement from the pet store.
Never give up on the injustices in our world. Bettas are remarkable little things, by returning him (even though he is in a pond now) you could have gotten another betta.. Not as a replacement but as another chance to help store bettas.
He likely had an internal problem, nothing an untrained customer could even notice. It is not your fault. You are a caring person, bless.