1) Amended provisions for set off of ITC are in the following order:
a) Set off IGST ITC first fully against IGST liability (if any)
b) Balance IGST ITC (if any) can be utilized in any manner to set off CGST & SGST liability
c) Set off CGST ITC against CGST liablity & SGST/UTGST ITC against SGST/UTGST liabilty.
d) Set off of CGST ITC against SGST/UTGST liability and vice versa not allowed.
2) When the interstate seller buys from you, s/he/it would be able to claim IGST ITC and the same needs to be completely set off against IGST liability of such buyer. If the buyer makes interstate sales, then he may utilize the IGST ITC (on purchases from you).
3) But if the buyer makes interstate purchase (IGST ITC) but sells predominantly within the state (CGST & SGST/UTGST liability), then such person can first set off IGST ITC against IGST liability (if any). Balance IGST ITC can be set off against CGST & SGST in equal proportions to get maximum benefit. If this is not done, then the buyer would end up blocking credit under one head (say CGST) and making payment from the other head (say SGST).
4) The link below provides examples to understand the impact of set off. If one has patience to read it completely, I think it would help to understand even better:
https://cleartax.in/s/gst-input-tax-credit-utilisation#amended
I know I've posted a lot but I've tried my best to keep it minimum. Hope this is not confusing.