(1) A wrongdoer who has paid, whether to the judgment creditor or by way of contribution awarded to concurrent wrongdoers, an aggregate sum in excess of the amount apportioned to him under section 8 of this Act shall be entitled to recover contribution from any concurrent wrongdoer who is not entitled to indemnity or who has not already paid as aforesaid a sum equal to that apportioned to him.

(2) The contribution shall be the amount of the excess or the amount by which the sum which the concurrent wrongdoer has so paid falls short of that apportioned to him, whichever is the less.