Each financial advisory relationship shall be evidenced by a written document executed prior to, upon, or promptly after the inception of the financial advisory relationship, or promptly after the creation or selection of the issuer if the issuer does not exist or has not been determined at the time the relationship commences. That written document shall set forth the basis of compensation for the financial advisory services to be rendered, which, except for bonds issued prior to January 1, 1988, to finance single-family or multifamily housing, shall be on a basis other than as a percentage of the amount of the bonds to be sold.