Example. A owns 500 shares of stock in the XYZ Corporation which he purchased prior to March 1, 1913. A is considering selling 200 shares of such stock but is uncertain as to the basis of the stock for the purpose of computing gain. Either prior or subsequent to the sale, a closing agreement may be entered into determining the market value of such stock as of March 1, 1913, which represents the basis for determining gain if it exceeds the adjusted basis otherwise determined as of such date. Not only may the closing agreement determine the basis for computing gain on the sale of the 200 shares of stock, but such an agreement may also determine the basis (unless or until the law is changed to require the use of some other factor to determine basis) of the remaining 300 shares of stock upon which gain will be computed in a subsequent sale.
However, a closing agreement with respect to a taxable period ending subsequent to the date of the agreement is subject to any change in, or modification of, the law enacted subsequent to the date of the agreement and made applicable to such taxable period, and each closing agreement shall so recite.
26 C.F.R. §301.7121-1