The teacher of God is a miracle worker because he gives the gifts he's received.

We will continue with the Manual for Teachers and the sections: Is Healing Certain? and Should Healing Be Repeated?

From Is Healing Certain?:

It is not the function of God's teachers to evaluate the outcome of their gifts. It is merely their function to give them. Once they have done that they have also given the outcome, for that is part of the gift. No one can give if he is concerned with the result of giving. That is a limitation on the giving itself, and neither the giver nor the receiver would have the gift. Trust is an essential part of giving; in fact, it is the part that makes sharing possible, the part that guarantees the giver will not lose, but only gain. Who gives a gift and then remains with it, to be sure it is used as the giver deems appropriate? Such is not giving but imprisoning.

From Should Healing Be Repeated?:

Whenever a teacher of God has tried to be a channel for healing he has succeeded. Should he be tempted to doubt this, he should not repeat his previous effort. That was already maximal, because the Holy Spirit so accepted it and so used it. Now the teacher of God has only one course to follow. He must use his reason to tell himself that he has given the problem to One Who cannot fail, and must recognize that his own uncertainty is not love but fear, and therefore hate. His position has thus become untenable, for he is offering hate to one to whom he offered love. This is impossible. Having offered love, only love can be received.