Sounds like they're trying to take advantage of your desire to get out of an old car and into a new car. They're giving you too little on your trade in, and charging you for the gas in the car!?
They'll negotiate more when they see that your serious. You should be able to get anywhere between 2-4% off the top of a new car (I got 3.5% off), and better value for your 323, and no fuel charge! Do your test drives, sit down and be firm, otherwise just go to another dealer and repeat the process.
The used car market it flooded, so it's tough to squeeze a dealer. If you can do it and you don't mind taking the risk, buy a new car and sell yours by undercutting the current offerings on autotrader. Might work
Or, just buy snows as others have suggested and save up for your next bimmer in a few years. I don't see too many xi models out there, doesn't seem like it's really necessary, IMO.