Should a disabled member be restored to active service, such member’s disability allowance shall cease. The disabled member shall again become a member and shall contribute thereafter at the same rate in effect prior to disability. Any prior service certificate on the basis of which the member’s service was computed at the time of retirement shall be restored to full force and effect and in addition upon the member’s subsequent retirement, the member shall be credited with all service as a member, and if the member’s then average final compensation is less than the average final compensation used in determining the member’s disability allowance, the latter amount shall be used in determining benefits.