Can I apply discounts on invoices after creating and posting them?

No. You cannot directly apply discounts to an invoice that you have already created and posted. However, you can use a write-off or credit payment to reduce the balance amount on the invoice. Note that this doesn’t show up as a Discount Applied field on the invoice like it does when you apply a discount during the creation of an invoice. Check out this article for details on offering discounts on invoice payments.