Gst on sale of asset

We have considered some mobile phones as asset in books now if new phone is purchase in exchange of old one than is there any GST liabilty on the amountof exchange done? as it was there in vat regime we need to pay Vat on the sales receipt. do we need to pay GST on sales receit? for Eg price of new phone including GST is 42900 & the exchange of old phone is Rs 4000 so is there any GST liability on 4000?