Whenever for any reason water system facilities are proposed to be installed in a district by someone other than the district itself, or the installation cost is proposed to be paid by someone other than the district itself, and the facilities are to be thereafter dedicated to the district for public use, and the board determines that it is necessary and convenient to the purposes of the district that the acceptance of said dedication be conditional upon the water system facilities being adequate to serve land other than and in addition to land proposed by the installer to be served thereby, the board may by contract agree to reimburse and may reimburse the installer for the proportionate part of the total cost of such water system facilities which will serve and benefit other land, upon such terms as may be agreed upon.