Watto owned both Anakin and Shmi, having won them from one of the Hutts. When Qui-gon did attempt to free both Anakin and Shmi, using the pod racer in a bet, Watto replies that no pod racer is worth two slaves. Qui-gon says just for the boy (Anakin) and Watto then comes back with the chance cube gamble, which Qui-gon neatly cheats. After the race Qui-gon did sell the pod racer for some credits, but Watto still owned Shmi and I doubt he would be apt to sell her to Qui-gon for anything less than a fortune.
QUI-GON : I'll wager my new racing pod against...say...the boy and his mother.
WATTO : A Pod for slaves. I don't think so...well, poerhaps. Just one...the mother, maybe...the boy isn't for sale.
QUI-GON : The boy is small, he can't be worth much.
WATTO shakes his head.
QUI-GON : (Cont'd) For the fastest Pod ever built?!
WATTO shakes his head again.
QUI-GON : (Cont'd) Both, or no bet.
WATTO : No Pod's worth two slaves...not by a long shot...one slave or nothing.
QUI-GON : The boy, then...
WATTO pulls out a small cube from his pocket.
WATTO : We'll let fate decide. Blue it's the boy, red his mother...
WATTO tosses the cube down. QUI-GON lifts his hand slightly; it turns blue.
QUI-GON smiles. WATTO is angry.
WATTO : (Cont'd) You won the small toss, outlander, bou you won't win the race, so...it makes little difference.