Created a new problem on the device

On Sept 14th I gave in my Samsung S6 edge to a samsung care store just for a minor screen repair, everything on the phone worked like normal, there was no other problems with the device. On the 22nd a technician called me to apologise for the delay with the repairs saying that they received a replacement screen but that screen itself had a fault and that they would try to have new parts and the phone ready by the end of the week. On the 29th another technician called me and said that they had repaired the phone just that the phone is working in black and white and assumed it was the settings and will get back to me later. I grew abit impatient and decided to call them back myself the technician said that the phones main board had been damaged and that i must pay for the new part as its my fault ,i told him i found that hard to belived coz they had done the checks and saw the phone was fine and it was only the screen he then insisted its normal with Samsung products and that i have to pay R3900 for the new board ,I find it hard to believe I assume the tecnicians damaged the device when they tried to replace the screen and now want me to pay CAN SOMEONE PLEASE HELP ME WITH THIS PLZ.