Bad service

I withdrew R1000 from a FNB atm using my Nedbank card. The atm only dispensed R900 with a paper written counter use only and FNB date stamp written cash centre jhb. I logged the incident cause I want my R100 it’s been two weeks now and im being sent pillar to post. FNB is sending me to Nedbank and Nedbank is telling me they declined my refund query and why I ask the Nedbank agent they telling me FNB didn’t give them a reason why they declining it. Firstly the FNB consultant asked me the ATM number and I told them I don’t know it he then asked why didn’t I print the slip so how are they going to see which atm I was using? That frustrated me worse cause firstly the atm didn’t have receipts it told you immediately when you inserting your card, secondly is it a must to print receipts when withdrawing? If yes then why is there option to choose whether you want the receipt or not. Problem I can’t post the paper here I received from the atm instead of my R100 note