https://marco.org/2009/12/15/the-sensei-the-master-teacher-was-not-charging
The sensei (the master teacher) was not charging the group for the training. The students, feeling that this was unfair, approached the master one day and suggested that they pay him for his time and effort. Setting down his bamboo shinai, the master calmly replied that if he charged them, they would not be able to afford him.