Is taking 50 pounds from some students considered an injustice to those who paid 60 pounds, making 10 pounds of it unlawful money, or does the teacher have a right from those who paid 50 pounds, and no unlawful money is claimed from those who paid 60 pounds?

A teacher's reduction of the fee for some students but not others is not considered an injustice, nor does it make a portion of his earnings unlawful. He has the right to receive the agreed-upon fee from everyone, and his waiver of a portion of this right for some students does not obligate him to waive it for the rest.