outrageous explanation for not repairing phone

i sent my cell phone in for repair due to audio fault and software update. I was just called by Priscilla of the repair centre telling me that my phone will not be repaired under warranty due to the fact that it has a small crack on the top corner of the phone and if i pay R900 for the screen to be replace then they will repair the fault which is actually covered under the warranty. Firstly i dont need my screen to be replaced as the crack is barely visible and secondly i dont have over R900 and especially for something i dont need to be done. She confirmed that the crack to the phone is not the cause of the fault so i find this absolutley outrageous that i have to pay for the screen to be replaced before they can repair a fault that was not caused me!!!!!!