1. 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.2. A grantor shall be allowed a credit against the tax due on a conveyance of real property under the following circumstances: (a) the grantor purchased the property without its being improved by residential premises, (b) the grantor improved it by the construction of residential premises thereon, and (c) the grantor subsequently sold such improved property. At the time of the first sale of the property after the making of the improvement, the grantor shall be allowed a credit equal to the amount of the tax actually paid on the conveyance immediately prior to the making of the improvement.