what is my tax liability

You have LUT so u need not to pay nay tax liability and in this case u have received 5lakhs so on that tax u can claim refund from unutilised ITC as per Sec 54(3) of CGST Act, 2017.

It is obligatory , on the part of exporter to realise payment for export of goods within 9 months & if the payment is not reced. within the prescribed limit, the exporter duty is to inform the Bank about this.
The exporter has to surrender. proportionate export incentives. hence also cannot claim refund for proportionate portion for unutilized ITC .