RSAWeb expects me to pay for their mistakes

I genuinely hate to give this company a bad review, because when I joined they were very friendly and helpful. The company has now made a mistake with my account by crediting me with R500 that they were supposed to credit someone else's account with. It was their mistake and was made in Oct last year. Now in February they are now holding me liable for their mistake and making me pay for it. I find it unacceptable that they expect me to pay this amount that they incorrectly allocated.