Myntra.com - Order cancelled without asking in prepaid order

On **rd june **** i have ordered * products of worth rs ****.** with phone pe as there was offer going on that if i pay via phone pe i will get a cashback of **% on orders above ****.so i ordered using phone pe and i got **% of my order value. after ** days i received * products of my order.now after ** days they have suddenly cancelled my order without even asking me neither they have told me about that .it took them ** days to know that their product is out of stock as the time of ordering many quantities were available.my whole money was with them for ** days and they dont bother to reply back .so now what they have done they are refunding only rs *** as they have deducted my whole cashback value of my total order value why because i am not eligible for cashback as my order value has gone less than **** .but the problem is that why should i bear the loss as i have not cancelled the order and i have not order less than ****.its totally their fault they should bear the whole loss.
order id - *******-*******-*******
please help me and resolve my issue as soon as possible .
thank you.