No bonds or other evidence of indebtedness authorized hereunder shall be sold for less than the par value thereof, plus accrued interest, and the proceeds from sale of all of said bonds and all money otherwise borrowed hereunder shall be paid to such county, independent rural, union high and municipal board of education or board of regents of said institution and by the treasurer thereof placed in a separate fund to be used and paid out only for the specific purposes for which the same is borrowed, and any amount left after erecting or purchasing any such building, improvement or facility or making any such improvement for which said money is borrowed shall be converted into the “interest and retirement fund” hereinafter created; but all costs incident to issuing and selling any bonds, or otherwise borrowing any such money, the making and delivering of any other certificate or evidence of indebtedness, including legal expense, may be paid out of the money borrowed and provided for herein; and the county, independent rural, union high and municipal board of education or board of regents of such institution at the time of issuing any bonds or other evidence of indebtedness shall establish for the payment of the principal and interest of such bonds or the repayment of all money otherwise borrowed a separate fund to be known as “interest and retirement fund,” into which fund shall be placed all net income from the use of any such building, facility or improvement erected, purchased or made with the money so borrowed, and the money so placed in said fund shall be used for the sole purpose of repaying the principal and interest of the money so borrowed, with any necessary service charges; and the issuance of any such bonds and other evidence of indebtedness shall constitute an irrevocable pledge of said county, independent rural, union high and municipal board of education or board of regents of all net income from the use of such building, facility and improvement for which such money was borrowed; provided, each separate building or group of buildings, facility and income-producing improvement erected or purchased and made hereunder shall be a separate unit and all net income therefrom used solely for the repayment of the money borrowed therefor, used in erecting or purchasing such building or facility and making such improvement.