Any serviceman required to collect the tax imposed by this Act shall be liable to the Department for the tax, whether or not the tax has been collected by the serviceman, except when the serviceman is relieved of the duty of remitting the tax to the Department by virtue of having paid a tax imposed by the Service Occupation Tax Act upon his or her sale of service involving the incidental transfer by him or her of the same property. To the extent that a serviceman required to collect the tax imposed by this Act has actually collected that tax, the tax is held in trust for the benefit of the Department.