Appauling service

My name is Ncumisa Mfo. on the 26th February 2019 my car went in for a major service in one of your branches (Port Elizabeth) before going for the major service my car had no issues what's so ever. The afternoon of the 26th Adam called me advising that my car is ready for collection. On the same day the oil sensor kept flashing on an off I called them again they said I must bring it in. On the 28 February 2019 I received an sms to rate their service which I response I told them that I stil have the same issue. Adam called me the morning of the 28th round about 6:45 advising that I should bring the car in "its our fault" those where his words on the call.I came in again on Friday the 28th February 2019 he then said the problem has been fixed if I have the same issue I must come in again. On Saturday 2nd March 2019 I had the same issue . This morning I took the car back now Adam is telling mt that I should pay R4800 they need to change the oild pump. His explanation to this is that the oil was old and damaged the oil pump. How is this possible when I serviced my car with them last for a minor service and came back within my service period to do a major service? and when I took the car to them my car had no problem what's so ever. I than called head office spoke to Vanessa to complain about this she called me back and advised that she has spoken to the P.E branch,basically she gave me the same information Adam gave me. Im really not happy about this before my car had no problem before it went to them.