How to make someone realize his mistake

Golden sunrise

This all strted from my last event where a vendor was recommended through a friend of mine for work . I gave him all the work n trusted him but on the other hand he goofed up everything n as a result I faced a huge loss . Bt now to my surprise we spoke for a certain amt bt inspire of all mess he is forcing me to pay much higher amt . He is not realizing his mistakes n feels he has done mistakes bt thts ok . Just coz he is related to my friend I can't spoil things . Plz suggest something so tht he agrees to my deal realizing his mistake completely n things gets sorted without any further complications . At this point of time I m unable to think what serums shld I use ... plz suggest something to solve this issue