The laws assume that the relationship between the contracting parties prima facie influence the price at which the transaction is being carried out. Since the relationship did not exist on the date the prices were finalized (i.e., entering into the contract), the transaction value should be accepted in case of supplies effected prior to the forming of such relationship. However, for supplies effected after the two persons become ‘related persons’ for the purpose of the GST law, the transaction value cannot be accepted and reference must be made to the CGST Rules.
(ICAI FAQ PUBLICATION 06-09-2017 VALUE OF TAXABLE SUPPLY : FAQ NO.55)