COLLECTION FOR ARREARS

I received a call few months back from a collection department St Johns re my Mr Price account. I paid them R500 as per arrangement. The following month I received a call from another collection department MBD who informed me Mr Price appointed them to collect arrears from me. St Johns told me Mr Price did not inform them that MDB must collect. I emailed Mr Price to asked for a proof on black and white to whom is handling my account. Up to date no response. I refused to pay MBD and St Johns. I would like to know what happened to the R500 I paid St Johns. I get calls sms' regularly from both.

Please assist