In case the University of Washington should attempt to use or permit the use of such shorelands or any portion thereof for any other purpose than for arboretum and botanical garden purposes, except as provided in RCW 28B.20.354, the same shall forthwith revert to the state of Washington without suit, action or any proceedings whatsoever or the judgment of any court forfeiting the same: PROVIDED, That the board of regents of the University of Washington is hereby authorized and directed to reconvey to the state of Washington block eleven-A (11-A) of the supplemental map of Lake Washington shorelands, filed September 5, 1916 in the office of the commissioner of public lands, or such portion thereof as may be required by the state of Washington or any agency thereof for state highway purposes. The state of Washington or any agency thereof requiring said land shall pay to the University of Washington the fair market value thereof and such moneys paid shall be used solely for arboretum purposes. Such reconveyance shall be made at such time as the state or such agency has agreed to pay the same.