• that's not playing the game
• finder will be rewarded
• fairly, justly, on the level, on the square, squarely, trustworthily
• fairness, frankness, honesty, honor, honour, integrity, probity, rectitude, righteousness, squareness, straightforwardness, trustworthiness, uprightness
• to do the right thing, to play a square game, to play the game
• to do the proper thing by sy
• honesty beyond question