discount details on invoice - Goods and Services Tax - GST

Sir, iam a electronic goods dealer, i purchase goods for dealer price and sell for less example: I purchase fridge for Rs.25,000/- from manufacturer and sell forRs. 24, 000/- to custmers. The difference amount is compensated by manufacturer in the way of credit note of Rs.1500/- at month end . ultimetley i had earned 500?-Rs. The questions are 1) ITC for 1,000/- got accumulated with me, should i reverse this back...to whom? 2) Is it mandatory to raise invoice for Rs. 25,000/- to customer and mention the discount offered Rs1000/- on the invoice .