No bill shall be passed giving any extra compensation to any public officer, servant, employe, agent or contractor, after services shall have been rendered or contract made, nor providing for the payment of any claim against the Commonwealth without previous authority of law: Provided, however, That nothing in this Constitution shall be construed to prohibit the General Assembly from authorizing the increase of retirement allowances or pensions of members of a retirement or pension system now in effect or hereafter legally constituted by the Commonwealth, its political subdivisions, agencies or instrumentalities, after the termination of the services of said member.