Undoing anonymous donations All the students in a class are planning to do a trip. Not all of the students can afford it, and it is considered shameful to reveal their poverty. So it is suggested that anyone can donate anonymously to a fund. If the fund becomes big enough to cover the trip, the trip happens. If not, the donators gets their money back, preserving anonymity of who donated and who didn't.

Is this possible?
Sure. Create a computer system where students can enter a password of their choice and then donate as much money as they choose. If there is not enough total money at the end, students simply re-enter their passwords and the machine gives back that much money.
Yes it is possible if the paiement is done via Paypal / Online paiement.

Bank transactions are validated and done only if the fund limit is reached. So, while the fund is not reached, nobody loose money and everybody stay anonymous.