Mtn disappointing

I have a cellphone contract with them , last month I requested them to change my debit order date from the 7th to monthend I then paid an amount of R430 next thing they sent me a message that they didn’t get funds in my account I told them that I changed the debit order date, monthend they debited R854 on top of the R430 that I paid week before I’m Thabo Saka ***