My Son, the Time Traveler (The Zack Files, #8)

After months of hoarding his allowance, Zack is on his way to buy a prized baseball card. But when he gets to the store, the owner tells him that someone else has just bought it. Who, you may ask, has beaten Zack out of the card? A boy named Mack, who claims to be Zack's son from the future.