Unhappy client

Hi, I recently bought a house through Cosmopolitan Homes and it is financed by Nedbank. I however changed my account to capitec and informed cosmopolitan of this ,seemingly they didn't contact Nedbank. Yesterday I was contacted by Nedbank and i gave them the new account details and made an arrangement that they should double-debit as i had missed my payment for this month.However as i realized that i wont be able to affordable that much money as it is R 12,200.I i called them today to make a new arrangement and they refused to assist me telling me that the arrangement has already been made,so does that mean arrangements cant be changed?