Can I give a discount on an invoice?

[afreshour]

I am sending an invoice to a customer who had a $100 credit she needs to use.  How can I insert a discount/credit on an invoice.  I tried to just enter a negative number but it didn't seem to work.  Any help would be greatly appreciated.  Thanks!

[surplusdealdude]

You can't give a discount, but you can manually reduce the price of some of the items by $100 and explain why in the comments section.