In the case of any county owned land within any district has been segregated into farm units as provided in RCW 89.12.040 and the appraised value thereof established, the board of county commissioners of the county shall have authority at its option of entering into a contract with the United States to bring any of such county lands as the county board shall determine under the provisions of the recordable contracts provided for in RCW 89.12.040, whenever such contracts are required as a condition to the delivery of water under the terms of the contract between the district and the United States, upon such terms as shall be agreed upon between the county and the United States: PROVIDED, That such contract shall not obligate the county to pay any district assessments levied against such lands except such, if any, as the board of county commissioners of said county shall elect to pay: PROVIDED FURTHER, That nothing herein contained shall be construed to deprive the district of the right to assess such lands, if otherwise assessable and to enforce the collection of the same in the manner provided by law.