Discount after taxable invoice raised

For  Discount  you refer  Section 15( 3)  of the CGST act  2017 

Actually its a practical doubt.
For eg: As per invoice Taxable amt=1000
Tax= 50
instead of paying 1050 they adjusted among themselves to pay without any credit note at discounted amt for 1020.
Entry passed: Purchase 970
Input tax 50
To Bank 1020
So they are arguing that as invoice is for 50 tax so even if discounted paid they can claim itc for 50
is treatment correct???

If you refer section 15(3)(b) of the CGST act 2017
, discount effected after the supply you need agreement entered into , so no aggement then this discount it self is question .