Section 1565. Credit

A buyer shall be allowed a credit against the tax due
  on a conveyance of real property to the extent  tax  was  paid  by  such
  buyer on a prior creation of a leasehold of all or a portion of the same
  real  property  or  on the granting of an option or contract to purchase
  all or a portion of the same real property by such  buyer.  Such  credit
  shall  be  computed  by  multiplying the tax paid on the creation of the
  leasehold or on the granting of the option or contract  by  a  fraction,
  the numerator of which is the value of the consideration used to compute
  such  tax  paid  which  is not yet due to such seller on the date of the
  subsequent conveyance (and which such seller will  not  be  entitled  to
  receive  after  such  date),  and  the denominator of which is the total
  value of the consideration used to compute such tax paid.