Money hungry peasants

Payment arrangements where communicated on the 7th of August, they continued calling me smsing me even though payments arrangements were made and there was no missed payment so they had no reason to contact me. To my surprise I settled the debt but now I owe and additional amount of over R1k, and I asked them how is it so, they say they will check they will verify. But I still receive smses and calls to say I will be black listed. I want this to be resolved or else this will turn into a legal issue.