The transaction value shall be accepted if the price is the sole consideration. In other words, what is received or what is to be received should only be the price paid or payable to the supplier. No other consideration in any form should be payable for the supply.
section 15 value of supply "price is the sole consideration"
Just take example exchange your old mobile phone with new phone and pay some cash also... Example of price not the sole consideration... Valuation as per rule 27....and take another look example where u purchased a new phone only paying cash or bank... It is price sole consideration