Any mortgage contingency clause included in a bond for deed or a written agreement for sale of real estate which conditions the purchaser’s performance on his obtaining a mortgage from a third party shall satisfy the provisions of section 52-550 if such mortgage contingency clause contains at least the following: (1) The principal amount in dollars of the loan the purchaser must obtain to fulfill such contingency; (2) the limit of the time period within which a commitment for such loan must be obtained, and (3) the term of the mortgage expressed in years.