A purchase of R844.35 was made without signature

My Woolworth store card had a buying balance of R4500 plus before my purchase, I purchase goods for R883.26 then when I got home I checked my balance the remaining balance was R2830. Then I realised that something is wrong then I called the call centre they told me that another purchase of R844.35 was made on my account that I was not aware or authorised. the call centre told me to back to Qaugga centre however when I went there they said they cant pick up the R844 purchase on their server, the promised to look on it and ask the teller for that day. then the next day they call me and say they still cant pick it up, I must call head office. When I called head office now they turned to say the don't know what happened now the R844 has disappeared from their system. I am so disappointed and action should be taken upon them. A ********** activity has happened on my account and know one what to explain.