Section 2C. Except as provided in section sixty-three A of chapter forty-four, whenever in any fiscal year the United States, the commonwealth, or a county, city or town, or any instrumentality thereof, or any entity whose real estate is exempt under clauses Third, Four, Four A, Fifth, Sixth, Seventh, Eighth, Ninth, Tenth, Eleventh, Twelfth and Thirteenth of section five, shall sell any real estate after January first in any year, the grantee of the real estate shall pay a pro rata amount or amounts, as hereinafter defined, to the city or town where such real estate is located in lieu of taxes that would have been due for the applicable fiscal year under this chapter if the real estate had been so owned on January first of the year of sale and, with respect to a sale between January first and June thirtieth, if the real estate had been so owned on January first of the year of sale and the preceding year. The pro rata amounts payable to the city or town shall be determined as follows:

(a) A portion of a pro forma tax for the fiscal year in which such sale occurred allocable on a pro rata basis to the days remaining in such fiscal year from the date of sale to the end of the fiscal year; and

(b) A pro forma tax for the succeeding fiscal year where the sales take place between January first and June thirtieth of any year.

The pro forma tax shall be computed by applying the tax rate or the appropriate classified tax rate of the city or town for the fiscal year in which such sale occurs, to the sale price after crediting any exemption to which the grantee would have been entitled under this chapter if the real estate had been so owned on January first of the year of sale.

Such amounts shall be paid by the grantee to the collector of the city or town within thirty days of the date of the issuance by said city or town of a notification of such liability to said grantee or the date by which a tax assessed upon real estate would otherwise be payable without interest for the applicable fiscal year, whichever is later. Any amount not paid by said date shall bear interest from said date at the rate per annum provided in section fifty-seven. The collector shall have for the collection of sums assessed under this section all remedies provided by chapter sixty for the collection of taxes upon real estate.

Sums received under this section shall not be subject to section sixty-three of chapter forty-four, but shall be credited to the general fund of the city or town.