The owner of what is known as a valid claim or improvement under the laws of this state, on public lands of the United States, shall be deemed in possession of a transferable interest therein, and any sale of such improvement shall be considered a sufficient consideration to support a promise: provided, that no such sale shall be valid to convey such improvement when made by the head of a family, unless the wife of the vendor, if any there be, shall give her consent thereto: and provided, also, that such land and the claim thereto shall be exempt from forced sale under execution.