Invoicing under GST

Tejas Bhayani (Student) (84 Points)

29 September 2020  
My friend is manufacturer of T-shirts. He sold 2 T-shirts for ₹1000/- with condition that if buyer pays the amount within 90 days, he will get extra 1 T-shirt at the time of payment.

How the invoicing will be done under GST ??

Plz help me....