And in the case of every decree of modification which shall be pronounced after the passing of this Act as aforesaid, it shall and may be competent to the said lords of council and session as commissioners aforesaid, and they are hereby authorized and required, to convert the said money stipend or money teind into grain or victual, save and except as aforesaid; and to make such conversion into grain or victual according to the fiar prices of the kind or description of grain or victual into which the same shall be converted, as appearing from the fiars of the county struck for each year in virtue of authority from the sheriff in which the parish shall be situated, upon an average of such fiar prices for seven years preceding the date of the decreet of modification, and exclusive of that year in which such decreet of modification shall bear date.