Describe the conditions to be satisfied for availing deduction of post supply discounts from the value of supply as per the provisions of section 15(3) of the CGST Act 2017.

Examine the existence of "consideration" for donation received by charitable institutions from individual donors, without quid pro quo an important feature as defined in section 2(31) of the CGST Act, 2017.

Mohan Enterprise is a registered person having principal place of business in Gandhinagar, Gujarat. They received services of Advocate Sameer, a registered person from Ahmedabad, Gujarat. Shekhar, an unregistered person provided services of labour to Mohan Enterprise. Explain the provisions relating to issue of invoice by recipient Mohan Enterprise if he is liable to pay tax under reverse charge under Section 9(3) or 9(4) of the CGST Act, 2017.