§ 1449-k. 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.