Is it true After he discovered that the frog had been weighted Smiley forced the stranger to give back the money?

No, that is not true. In the story "The Celebrated Jumping Frog of Calaveras County" by Mark Twain, after Smiley discovered the frog had been loaded with shot, he was upset but did not force the stranger to give back the money. The stranger left town before Smiley could challenge him.