Cal. Gov. Code § 66480

The public agency for whose benefit an area has been reserved shall at the time of approval of the final map or parcel map enter into a binding agreement to acquire such reserved area within two years after the completion and acceptance of all improvements, unless such period of time is extended by mutual agreement. The purchase price shall be the market value thereof at the time of the filing of the tentative map plus the taxes against such reserved area from the date of the reservation and any other costs incurred by the subdivider in the maintenance of such reserved area, including interest costs incurred on any loan covering such reserved area.