FAILING TO CORRECT ERROR

I have had my Markham account for as long as I remember. My limit was R6,636.00 and was then increased to over R8000.00. I phoned in as I wanted to decrease my Mr. Price account with a limit of almost R20,000.00 to R4000.00 and I did not realize that I am actually busy with the Markham account. I then phoned Mr. Price after realizing and they have decreased the limit as I hardly ever use the account, and I contacted Markham again to fix the error, in less than 48 hours of realizing this and they have advised that this cannot be done. Please tell me why it cannot be done as I have admitted that it was an error, I did not want to increase the account, I just wanted my account limit corrected to where it was before I made the HUGE mistake and now nobody is able to assist me. How is this possible?