Lut (letter of undertaking) in gst

Dear Sir,

We are into construction industries based in chennai. We supply materials as well as providing consultation services to our client in India and Abroad.

The problem is we have raised service invoice (April_19) to our client @ Africa without payment of IGST prior to submission of LUT. Later we realised and applied for LUT through online portal during June_19.

My question is...

1. Whether we are liable to pay IGST along with penalty for the invoice raised prior to the submission of LUT.

or

2. we can enjoy the exemption, Since the LUT applicable is from April_2019 to March_2020

Regards

Balachinnadurai.S