Tahoe Time-out

Tahoe found a green ball at the dog park on Friday.
He has decided it's a finders/keepers sort of a deal.
I believe it's one of the other dog owners who doesn't care to lug the ball to the dog park every day.
With some difficulty, I've convinced Tahoe to leave the ball at the dog park each time.
The first thing he does on arrival is find the green ball again.
He totally ignores the ball or Frisbee we've brought with us.
I'm going to be in serious trouble if the owner fails to leave the ball or if the owner is not good with sharing it!