135.357.  Capital gain exclusion, when. — A taxpayer shall be allowed to exclude from taxation under chapter 143 a portion of the capital gain, as calculated under the Internal Revenue Code of 1986, as amended, that results from the sale of a low-income project subsidized by the federal Department of Housing and Urban Development to a nonprofit or governmental organization, agreeing to preserve or increase the low-income occupancy of the project.  For those owners whose low-income project has at least forty percent of its units occupied by persons or families having incomes of sixty percent or less of the median income, the exclusion shall equal twenty-five percent of the capital gain.