Credit limit decrease

I have a credit card with Nedbank my limit is 12500 now I made a payment of 3800 on the credit card I want to decrease my limit to 9000 when I call the call centre am told am unable to decrease it how come because when I took the credit card my limit was 5000 now how is it impossible to go back to that limit Am under debt review I want to increase my credit score so am able to apply for car with you guys again I have ref numbers from your guys *** *** I went to the branch just now again they are unable to help me . Call centre guys are the worst they need to go for training