Reference for Bekhorot 91:4

וה"מ לדבר הרשות אבל לדבר מצוה יש לו הפרה

a case in point being that of a teacher whom R'Aha prohibited by vow from teaching any longer because he maltreated the children, but whom Rabina reinstated, as there was not to be found one who taught so efficiently.

Jastrow

Jastrow