Section 1449-FFF. Credit

A grantor shall be allowed a credit against the
  tax due on a conveyance of real property to the extent tax was  paid  by
  such  grantor  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 grantor.
  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 grantor on
  the date of the subsequent conveyance (and which such grantor  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.