I've seen enough of gamble's posts now to feel confident that he is a VERY customer oriented tractor dealer. Let's say that you are lucky enough to be dealing with the same kind of dealer in your area.
If that is the case and you have put all 30 of these hours on your 48" before learning that no mower attachment is available, then if you are willing to give up the 4" of cut width and the few horsepower that go along with it, the difference in the price of the 44 and 48 is about the same difference as the bagger attachment I would think.
If you approach him, as gamble recommends, in a positive, friendly and professional way, and if he still has an '04 Z-Force 44", maybe you can pay him a little for the hours use that you've gotten from your 48" to make the trade.
If you approach him with respect and no anger, you will have your best chance to work it out. By all means don't imply that he intentionally told you a lie. Indicate that you are sure that it was an honest mistake. You can always catch more fly's with honey than with vinegar.
There is no way to know until you talk to the man. If he is a customer oriented business man, like gamble, that realizes the value of happy customers, I'm quite sure you can work this out to your satisfaction.
It won't cost anything but a little gas and time to go find out. Do it in person, not over the phone. Clean your unit up and make it look as much like new as you can and take it with you when you talk to him.
Good luck,
Doc