With due respect, I am unable to agree with part of reply of my learned colleague Shri Padmanatham Kollengode Ji (i.e. portion which states ''If a single invoice is exceeding Rs.20 Lakhs is raised (say, a single invoice for Rs.25 Lakhs) then entire Rs.25 lakhs would be liable to tax ....').
For this post, I am dealing with a situation where supplier who is unregistered and who was otherwise not to be registered under GST and his obligation to get registered under GST is triggered when he supplied goods / services & raised a single invoice exceeding Rs. 25 Lacs against one / singular supply.
As per Section 2 (107), “taxable person” means a person who is registered or liable to be registered under section 22 or section 24.
Relevant portion (from point of this ongoing discussion) of Section 22 (1) reads as follows:
Every supplier shall be liable to be registered under this Act in the State or Union territory, other than special category States, from where he makes a taxable supply of goods or services or both, if his aggregate turnover in a financial year exceeds twenty lakh rupees:".
In my view, at the time of supplying goods / services (involving value of Rs. 25 Lacs), supplier's turnover was below threshold limit u/s 22. Only when he supplied goods / services with value of Rs. 25 Lacs, supplier's turnover got above (i.e. exceeded) the threshold limit u/s 22, warranting him to get registration under GST.
In other words, at the time of making subject supply, supplier was not a taxable person u/s 2 (107). And if so, Section 9 does not come into picture at all in my view and consequently, no tax is payable under GST on entire value of Rs. 25 Lacs.
P.S. Considering risks involved in above argument (For example, what is one / singular supply?), the supplier should take practical call (including charging & paying taxes on value exceeding threshold limits or even following up with views of my learned colleague Shri Padmanatham Kollengode Ji) depending upon factual aspects involved, risk appetite & so.
These are ex facie views of mine, the same should not be construed as professional advice / suggestion & I respect contrary views.