Account payment

Good day, last month on the 9th of october I paid2500 on my account, this month when I paid my account it showed that I only paid 1500, my receipt says 2500 and their system shows 1500, I paid cash and was given a written receipt, when asking the manager to fix it ,she said it will take a week, now a week later she tells me it is under investigation, meanwhile it has a negative input on my credit score, even lodging a complaint with customer care,nothing has been resolved, where did my money go, why cant they fix it, I wiil never do business with lewis ever again. I am considering taking legal action againts them, this is unexceptable,