"It was too late to take it back to the store because the store had closed already, and how would it look if I now take back the ball to the store and explain to them what happened? Are they going to believe me or not?"

Since you now realized it, I think that the best move would be to take the ball back to the store, and explain the situation to the owner of the store.
I am sure that the owner would believe you.
Why? Because you are a grown-up man (adult), not a kid, and you bought dog food from the store. You have no reason to steal a ball. This was clearly accidental.

If you keep it with the knowing that it belongs to someone else, then you clearly/obviously become a thief. Especially, if you know who is the person, and you also know that you could give it back to him/her.