Municipalities or counties may spend funds from any lawful source or pay for all or a part of the capital improvements or facility expansions to reduce the amount of impact fees. A developer and a municipality or county may agree to offset or reduce part or all of the impact fee assessed on that new development, provided that the public policy which supports the reduction is contained in the appropriate planning documents of the municipality or county and provided that the development’s new proportionate share of the system improvement is funded with revenues other than impact fees from other new developments.