I made the mistake of verbal scope of work on a recent job - mainly because I had worked with the customer many times before, but this time there were partners involved.

A bunch of extra work comes along outside the scope of the verbal agreement, so now customer/partners are disputing the invoice I gave them for these extras, claiming it was work that was agreed upon in my initial verbal quote.

This is Canada, so laws will be different but any words of wisdom on how to handle this situation?