increase or decrease in purchse value

RAJU MEDAR (ACCOUNTANT) (110 Points)

14 June 2018  
dear sir

one of client doing import purchase, on that time paying custom and igst asper customs BE Copy.

they clearing goods from customs on Proforma invoice received from party.

incase there is any changes in final invoice value ( increase or decrease in rate ) after customs process is completed.

is GST and custom need to pay on differential value??