פירוש על קידושין 33:3

According to the baraita we learned yesterday, it would seem that if the slave is sick and misses time during which he could work, he still goes out free at the seventh year. But according to the second baraita, this is true only if he works part of the time. If he is sick all six years, he does not go free.

If he is able to do some minimal work while he is ill then he need not make up missed time. But if he cannot do anything, then he must make up the missed time.