Is is possible to prove you possess an amount of BTC without actually spending it?
You can sign any text with the private key you own and anyone can verify that with a public key associated with a given Bitcoin address. As long as you make the message relevant to the conversation, say "This is LethalFractal, at 2013-09-07, I claim ownership of address 1234567890... as a proof for negotiations with John Smith", the person can know that it is in fact you owning a given address, and since they can check the balance of that address, you will have proven that you have access to that money.