There are many unanswered questions here, but I have several observations and (perhaps) a suggestion.
1. You said that there is new management; do you have the names of the prior management? If so, it would be useful to contact them (in a friendly and non-confrontational way) and asked them if they recall the facts as you do. If they do, and are willing to testify if needed, it's a huge difference; if they do, but don't backup your story, it becomes less appealing. Either way, you need to know.
2. Does the FBO have published pricing for the services for which you are being charged? Do they have signs up, is it on their website, anything like that? And, if they do, when with the first published?
3. Ultimately, the most important thing is to get your plane back. If a friendly discussion does not yield either releasing the plane for the originally-agreed charges, or reduction to an acceptable number, I would also be in the camp that says you should just go ahead and pay with a credit card and, if you believe the facts supported, dispute the charge with the credit card company.
In any event, you need to get the plane out, because the whole time were talking about this, more charges are running up.
If you fight this or if you do any of the too-cute things people have recommended, they’ll file a lien against the airplane and it’ll cost you more than $1200 to fix it. Pay the money and move on.
It would be useful to know what state all this is taking place in; imposition of liens against aircraft is regulated by state law; the registration of liens in the FAA aircraft records in Oklahoma is only a notice function, and has no legal effect in itself.
In Texas, for example, in order for such a lien to be valid, the party claiming a lien against personal assets like an aircraft must have actual possession of the aircraft; hence, another reason why you should do what's necessary to get the airplane back in your hands.