When any capital has been paid in, in property instead of cash, the valuation placed upon such property, in estimating it as paid-in capital, shall not be conclusive in estimating its value under the foregoing provisions, but may be disputed by the municipality and, if shown to have been excessive, may be reduced by the authority fixing the price of the plant and property as hereinafter provided. No municipality shall be obliged by the provisions of this section to buy any apparatus or appliances covered by letters patent of the United States or embodying a patentable invention, unless the complete right to use the same and all other apparatus and appliances necessary for such use is assigned or granted to such municipality at a cost as low as the cost of such right would be to the corporation whose plant is purchased.