breach of contract

Hi , I'm currently under debt review with NDA and last month I received an email saying my monthly installments will go up , soon after that I received an sms saying I was supposed to pay R4502 which was R2 more that the usual R4500 , I deposited R4550 to their account , now last week I then received a statement saying I only paid R4450 , I immediately emailed my proof of payment which clearly showed that I paid R4550 to 3 people who I usually communicate with but there was no reply nothing what so ever , so today I just received my monthly statement and it reflects that I only paid R4450 , I'm not sure if this is a typo or negligence , I've been trying to get ahold of NDA with no luck , please assist me in this regard.