Is it permissible to retract a vow to donate competition prize money to Palestine after winning it and being in need of it, or must the vow be fulfilled, and the money be considered unlawful if taken?

If you vowed to donate what you win from a competition to the people of Palestine, then you must fulfill this vow and give the money to them. You are not permitted to benefit from it, because the vow is valid and must be fulfilled. However, if the competition involves gambling, then the money is originally unlawful, the vow is not binding, and you must dispose of the money. The people of Palestine have a greater right to it.