GST on supply during warranty period

XYZ(Manufacturer) supplying goods to customers on payment of GST. Subsequently, goods are supplied on FOC basis, 1) During warranty period if found defective or damaged or 2) After warranty period if found defective, damaged or short supplied. a) Whether XYZ is required to pay GST on goods supplied on FOC basis under both the situations i.e. before and after warranty period? b) How to arrive at the value for FOC supply c) Whether proportionate ITC is required to be reversed if GST is not required to be paid on FOC supply?