(a) any transfer, mortgage, or contract which, if made between private persons, would be by law required to be made in writing, and any document required to be executed on behalf of the society, may be passed, received, or executed on behalf of the society, in writing, either under the common seal of the society, or under the signature of 2 trustees and the secretary or 2 members of the committee and the secretary, and any such contract may in the same manner be varied or discharged;