(4)     any other amounts that are based upon the rate of interest of bonds of the public body.   

J.     Any payments required to be made by the public body under the contract may be made from money pledged to pay debt service on the bonds with respect to which the contract was made or from any other legally available source.   

K.     Any contract entered into by a public body pursuant to this section shall not impair the contract of that public body with, or impair adversely, the owners of bonds issued by that public body.