section 76

Section 76(9) of which Act, please mention clearly..

CGST ACT, 2017
section 76, where any person who has collected the tax amount and not paid to the govt. has to forthwith pay to the govt whether supply taxable or not

If the supply attracts GST, the amount paid to Government can be adjusted against those supply.

If supply not attracts GST then the surplus amount available will be credited to the Fund or refunded to the person who has borne the incidence on such amount.

When tax is collected and not paid to the govt. then the person shall pay it forthwith(whether supply was taxable or not) to the govt. otherwise u/s 76 a notice will be issued and penalty will be levied.
Now under which section penalty will be levied??
u/s 122 penalty will be levied when person fails to pay beyond 3 months.

Also u/s 73 penalty u/s 73(9) will be levied if self assessd tax is not paid or "any amount collected as tax" has not been paid within 30 days from the due date of payment of such tax.
So how 73 is different from 76 on the basis of line in sec 73 i.e. any amount collected as tax

please provide your opinion on this too.

It is upto proper officer decision to fix the penalty after obtaining the reason that why the amount mentioned in the notice should not be paid by him to the Government..

Can PO issue notice u/s 73 i.e. any amount collected as tax but not paid instead of 76?

Yes.. PO can issue notice u/S 73 for tax not paid or short paid. But the reason for serving notice under this section should not be in the following reasons
1. Fraud or
2. Suppression of Facts
3. Any wilful-misstatement