Whenever a school district receives or has received an apportionment under this article for the purchase of a site which contains existing improvements, the board may require the district to dispose of any existing improvements that a condition of receiving an apportionment in any manner that the board deems proper. Whenever a district sells, leases or disposes of any site acquired under an apportionment or any improvements appurtenant to any site so acquired it shall contribute a portion of the net proceeds therefrom or the value of any consideration received

therefor, in reduction of any apportionment, such portion being proportionate to the state’s participation in the project.