1. Go to Setup, Settings, Customer & Sales (Receivables), then Discount.
2. Enter the percentage of the discount.
3. Enter the number of days you want to apply for this discount.
4. Enter the number of days when the net amount will be due.
5. Select Calculate early payment discounts before tax if you want to apply the discount before tax or leave blank if you want to calculate after tax.
6. Select Linked Accounts.
7. Select an account for Early Payment Sales Discount.
8. Select OK.
9. Open the Receipt window in the Customer & Sales module.
10. Create a new receipt.
11. You’ll have new columns for Discount available and Discount Taken.
12. You can apply the discount as required.

I want to give a discount by amount on a sales invoice