Yes if he is not going to get the flat registered now there are chances that he can receive notice from income tax department.
Also there was a mistake in your last question this will not be LTCG this will be business income for builder and therefore income will be calculated differently.
By reading your questions title I also got carried away and didnt notice that he is a builder.
There is no surety as this is his business income and it all depends on what business income he shows and other details.
If the amount would have been above the limit set by government then there were more chances of him getting a notice but here there are less chances.
If you want any further discussion you can always call.
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