doing people in

I needed money for medicine for my child. I've never dealt with buy back but was desperate. Friends used the Primrose branch and had high praise so I used the Alberton branch to put my Samsung galaxy s5 on buy back. I only wanted R500 to cover the medication and declined when I was offered more. I've paid the interest every month on the 18th without fail. Yesterday we had car problems so went first thing this morning. The manager informed us that the phone is already on the floor as the new interest payment date is the 16th! I never took notice of this and it wasn't verbally communicated either. How can they reduce the payment date by 2days? The manager then, with a smirk, sarcastically offered we buy the phone back for R2100. I'm gutted and extremely upset. We are sparking trying to get my phone back but I have never felt this helpless and taken advantage of. My first and last dealings with any cc. I understand it's a business but to change the due date and then mark it up with 300%, not to mention the manager that found the entire thing amusing.