I need help ASAP with Railway Furnishers

Hay I need help I have paid my account for Railway Furnishers on the 25th of February ,I paid R600 separately VIA "Owzo" online, as I always use it then they called to tell me that they only received one payment which is R300 so they requested proof of payment and bank statement from my Bank. I did provide them with everything they requested me to send, but even today they said i should pay them again because they cant find the other R300 which they claim is missing. What must I do now because it cant be reversed? They should involve the third party which is "Owzo" because I called my bank and they did assist me with all the proof they need, Now they harassing me with calls every day with no solution from them, Wes scenario is that the head office can't do anything with this to assist. PLEASE HELP ME ASAP