Any lease of state lands for grazing or agricultural purposes and any contract for the purchase of state lands may be assigned as collateral security, with the approval of the commissioner of public lands; and after such approval such assignment shall have the effect of giving the assignee a lien on any lease or purchase contract so assigned, or any renewal or renewals thereof by assignor and all rights of renewal of any such lease, together with the improvements thereon, to secure the indebtedness specified in such assignment and any further advances or expenditures authorized to be made by the assignee by the terms of such assignment; and after any such assignment shall be approved by the commissioner of public lands, and while same is in force and effect as hereinafter provided, no relinquishment or assignment of a lease or transfer of a state purchase contract, or portions thereof, embraced in such assignment shall be accepted or approved for filing respectively by the commissioner of public lands, unless the holder of such collateral assignment shall release in writing any collateral assignment held by him covering such lease or contract being transferred, assigned or relinquished or, in case of assignment only, unless the assignee agrees in writing to assume or take the lease or contract subject to the rights of any collateral assignee. The preference right of renewal of any lease held under collateral assignment shall be vested in the holder of such assignment, subject only to the right of renewal of the lessee at the date of expiration of said lease, and to all the provisions of law now in effect or hereinafter enacted; provided, however, that any renewal lease issued to subsequent collateral assignees, under the preference right of renewal provided for herein, shall be subject to the rights of the holders of the prior assignments of record in the state land office.