Question about old mobile exchange

Yes they mention the old mobile in invoice but they did not mention IMEI number of old mobile in invoice.
I purchased S21fe with exchange of redmi note 9 Pro.

I think their exchange partner is Cashify. So an pickup order is automatically booked and once it completes, cashify issues an invoice that the user sold their device to them. That invoice contains the old phone IMEI number.

None of the customer care knows this process and it took almost 2 days to find out this process.

Reason why so specific is that the user will not be liable if the old phone falls into wrong hands. The receipt issued by cashify serves as a proof.