Non-contractual billing & non-compliant install

As a director of a commercial property I accepted a quotation on the 27th of November 2014 for ugrades to the exisiting alarm system in the name of AFM Properties (AFM). 50% deposit was paid for the work to proceed. The work was done by the 8th of December 2014. We were told that the system was tested and is working 100%. During an inspection thereafter, by an electronic engineer, it was found that certain curtian beams has not been installed according to specification and were unable to protect the windows at all. It was reported, they admitted poor workmanship and rectified it. A few weeks later an outside beam became defective - MonitorNet was claiming that ants invaded the beam therefor AFM Properties are responsible to replace it. In the mean time they installed a loan unit. After discussions they sent the beam to an independant verifyer that concluded that it was probably a factory fault. On the 24th of March they emailed me to confirm the loan unit was a brand new one and there would be no need to replace it. On the 2nd of April Lime-Chem, the tennant informed me that an amount to the value of R7193.00 was deducted from their bank account i.r.o above. The question is why ?