This section does not affect any agreement existing before October 1, 1997, between a county and a municipality concerning the imposition of development impact fees. If a county imposes a development impact fee on new residential construction to finance the costs of school construction, a municipality shall assist the county by: collecting and remitting the fee for new residential construction in the municipality to the county; or requiring the fee to be paid to the county in accordance with the county development impact fee law or ordinance. The application of any impact fee paid under subsection (b) of this section shall have a rational nexus to the project for which the fee is assessed.