Is contract price not sufficient to determine valuation of supply?

Contract price is more specifically referred to as ‘transaction value’ and that is the basis for computing tax. However, the transaction will not be accepted as the value of supply where the supply is between related persons (including different registrations of the same PAN and principal-agent supplies), or where the consideration payable is not wholly in money.

(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 43)

How do we compute GST liability on supplies which are liable to tax under reverse charge?

  1. Separate provisions had been provided under the erstwhile service tax laws to
    determine the service tax payable on reverse charge mechanism which is linked to date of payment to the service provider unless the payment is made within 3 months of the date of invoice.
  2. However, such provisions are not there in the CGST Act, 2017. Accordingly, general provisions relating to change in rate of tax shall be applicable in
    determining the appropriate rate of tax

(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 35)