Disgusted by unnecessary Interest charges

During December Getbucks send out text messages confirming debit order dates to be done as people are getting paid eraly in December, I did reply to inform them of my payday and also send an email confirming when I'm getting paid. To my surprise I was debited before I got paid even though I did make them aware. I called their call centre requesting them to reverse the money as it was meant for other personal commitments, I also did forward the email that I confirmed my payday and debit order to be done then. I spoke with several consultants on that day finally the debit order money was reversed and the last consultant that I spoke with who called me back to confirm reversal has been done did confirm the date I confirmed im getting and that debit order is loaded for that day. Getbucks did not debit me on my payday as confirmed. When I realised that they did not debit me it was in January. They did not communicate with me concerning this. Now I'm been charged interests of R504 even though they are the ones that did not debit me as agreed. I have asked them to listen to the calls between myself and the consultants and also send me copies of the voice call recordings. All my emails were answered rudely. I have never skipped any payment and made sure Getbucks is aware of my payday for them to debit me, why am I charged interests as if I skipped my payment. Getbucks did not do their job and its unfair for me to pay for heir own mistakes. Where do they think I will get R504 to pay for interests?