Tell them you are sorry but no discounts are available.
Did the message come directly to you or via the eBay message centre?
In any event it's clearly feedback extortion, if the buyer gives you a neg you should be able to have it removed. I say "should" because nothing is every 100% certain.
What has happened since you received the message?

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.