Employee ignoring his job

I enrolled for short program in office administration in 2015 and I was told that I must pay my instalments by debit order and my banking details were taken at I told them when to debit. The first month they stroke before the set date and it bounced, then I had to go and pay cash,the second month the debit order went through. After the course was complete I went to fetch my certificate I was told that I'm owing and they refused to give me the certificate, I then went to draw my bank statement and give it to Tebogo who is the cashier,He saw and told me that he will fax it to the head office as there is no record any debit order or any of my banking details on their system then I was given my certificate. This year 2018 I received a call from the college saying I'm owing , I went to the college and Tebogo said he didn't fax the bank statement and that he misplaced it and it's Impossible to find it as they have moved to a new building. I told him to follow it up and he told me the system is off line and but he will follow it up . Today I got a call from their attorney saying that I'm owing and the certificate they gave me is invalid and that it's just a copy. Now I'm going to be black listed because someone ignored his job.