I am not au-fait with the whole situation regarding buying back service. However, I do know it is expensive to do so and payments include compound interest which is crippling. Another thing, if you brother is retiring on grounds of ill health he is be entitled to added service as a result (calculable on all his "pay service" which includes his Half-Pay periods. The Superannuation (or Pensions) Section of where your brother is employed will be able to inform him of all his entitlements. Therefore, I would advise him to contact that section asap where he will acquire accurate information and probably fairly fast.