As Gail said, you can do it verbally, present it that is. In order to protect the seller, the buyer and the broker, an agent should deliver an offer in writing to the seller, regardless of who the agent represents and who's buyer it is. People can get pretty nasty if the deal turns sour and you have no proof that the offer existed. 5 minutes before I read this I just negotiated an offer verbally. If someone else comes along before it is signed by both parties, the first could lose out.
Let me ask YOU a question. Let's say you applied to a company for a job 500 miles away that paid $20,000 per year more than your current job that you have been in for over 5 yrs. This new company offers to pay all your closing and moving costs and will also give you a sign-on bonus of $10,000.
Would you quit your current job without getting the new offer in writing?
I rest my case!