No-lose scenario for Oakland. If Manny can still hit, they've signed a very valuable asset for just $500K. If he can't hit, they'll find that out in the minors when he's serving his suspension, and cut him without even paying his full salary.
"It breaks your heart. It is designed to break your heart. The game begins in the spring, when everything else begins again, and it blossoms in the summer, filling the afternoons and evenings, and then as soon as the chill rains come, it stops and leaves you to face the fall alone." --- Bart Giamatti, on baseball
He bet on games he was playing in and/or managing. When you lose the integrity of the game, you lose all. He knew betting on baseball would get him kicked out of the game, yet he did it anyway. He signed the document.