they billed me for service dey didn't render

I bought a decorder package worth R799 on the 26/01/2015 then they installed it the next day.3 days dwn de line it was off then wen I queried it dey said dey gave me a wrng decoder I brought it bck nd it started wrkng den after dat dey said I should pay R270 nd asked wats gona happen if I dnt dey said dey will cut it off nd dat wat happend but last on de 25th dey sended an sms dat I owe R600 nd something nd wen I asked y dey said it was wrking eva since nd am liable