CGST/SGST paid when IGST was payable and vice versa :
A taxable person who has paid CGST/SGST/UTGST (in SGST/UTGST Act) on a transaction considered by him to be an intra-state supply, but which is subsequently held to be an inter-state supply, shall, upon payment of IGST, be allowed to take the amount of CGST/SGST/SGST (in SGST/UTGST Act) so paid as refund subject to such conditions as may be prescribed – section 77(1) of CGST Act.
Luckily interest will not be payable- section 77(2) of IGST Act.
Parallel provision is made in IGST, which reads as follows –
A taxable person who has paid IGST on a transaction considered by him to be an interstate supply, but which is subsequently held to be an intra-state supply, shall, upon payment of CGST and SGST/UTGST in the appropriate State, be allowed to take the amount of IGST so paid as refund subject to such conditions as may be prescribed – section 19(1) of IGST Act.
Luckily, interest will not be payable- section 19(2) of IGST Act.
Provision of unjust enrichment will not apply to such refund – section 54(7)(d) of CGST Act.
Unfair provision – Really there should be adjustment between Central Government and State Government in such cases. That will be fair to taxable person. If the recipient has availed input tax credit, refund will not be admissible. This will cause double jeopardy to the taxable person. At least IGST and IGST could be adjustable as both are with Central Government.