"Is there any way the district could force the principal to give the money to the PTA if the funds are "donated" to the field trip fund?"
That would be extremely unlikely. Why would they?
There's nothing wrong with the parent group giving funds to the school from an accounting/IRS angle. There shouldn't be any reason the school or district would be concerned. What the PTA unit does with its funds is completely within its jurisdiction as long as it is active (and as long as the expense supports its non-profit purpose). Once the funds are given to the school, they belong to the school.
If you have any concerns, then continue to spend it until it's all gone. I'm sure the principal can give you you a laundry list of supplies needed.
I think the field trip fund would be fine. Are there any traditional expenses in next few months. A DJ, for example, or a school performer who comes in to school? Either might gladly take a prepayment. Something like that.
We are trying to change from PTA to PTO. We have our PTO established. We would like to quit the PTA, but we still have funds. What is the best way to get rid of that money before we disband? It's not very much, but on principle we would like to not have it collected from the local PTA. We had a Halloween event last week and spent a lot of it on food, which is now consumed of course. Could we donate the funds to the school? Could we donate it to the teachers field trip fund?