In every case where a partial redemption is asked for, pursuant to section 140.390, the county collector, upon the application of the redemptioner, after notice to the holder of the certificate, shall determine the proportion to be paid by the party applying to redeem, and his decision shall be final thereon. For his services in stating the proportion, the redemptioner shall pay him fifty cents; and in every case of a partial redemption, pursuant to said section, the quantity sold shall be reduced in proportion to the amount paid on such partial redemption, and the county collector shall convey accordingly.

(RSMo 1939 § 11144)