Query regarding Taxability of Contingent Income billed in Advance

Hi,

How do we treat the following:

There is a government contract for which payment terms are as under:

80% of bill payment agaianst installation

20% of bill payment as 2% per annum for 10 years on successful maintenance of asset

Putting numbers for better perspective:

Billed 10 Cr in 2020-21

Payment rec'd 8 cr in 2020-21

Rs. 20 lac receivable each year from 2021-22 till 2030-31

Now as total bill is raised gst we pay in current year 2020-21 on entire 10 Cr.

However, how should Rs. 2 cr be treated in terms of profit and loss account as well as income tax there on, considering that this income/receipt is contingent on performance of future years

Paying income tax of 30% on this 2 cr will be huge burden as it income not earned at all and money will also be earned in future by performing service.

Can any provision be created which is safe from income tax.

Thanks a ton in Advance