Money withdrawn from my account

Standard bank sent me an sms notifying me that due to an interest rate hike my mortage bond repayment my new intalment will go up by a certain amount. That amount however was around a R1000 more than what is should be - A MISTAKE THEY ADMITTED TO HAVE COMMITED AND SET TO RECTIFY. Instead of processing a refund (close to 3 weeks later) THEY ARE REQUESTING THAT I WRITE A LETTER OF MOTIVATION TO GET MY MONEY BACK INTO MY ACCOUNT. How does someone take your money, which negatively affects your disposable income and tell you to motivate them to give it back to you ??? I HAVE BEEN BANKING WITH STANDARD BAND FOR AROUND 10 YEARS BY THE WAY, I PAY R325 MONTHLY BANKING FEES AS A PRIVATE BANKING CLIENT ANT YET... THIS ???