Dispute over payment of a document translated from English to French

I was translating documents from English to French and French to English for an organisation from 2015 onwards. I also teach French there. On April 1st, 2018 I translated a legal document from English to French and it was accepted (I had been translating similar documents since 2015). On April 8th, 2018 I translated a similar legal document from English to French and the corrector said that because there were many translation errors she had to redo the whole document and that I would be paid only 30 % of the translation fee rather than the agreed 60 % which was due for each document. Since I did not agree with the decision both about the translation errors and the reduction in the payment because that would set a precedent for the translation of other subsequent documents, I had no choice but to resign from the translation service. How can I legally challenge the decision about the reduction in payment for the document?