Is it permissible to retract a land donation designated for building a Qur'anic memorization school in order to help the husband, or to sell it and buy cheaper land, knowing that the beneficiaries were not informed of the land's location and it has not yet been handed over?

If it is merely a promise of a gift, then nothing is binding upon you. However, if you are able to sell this land and purchase another to gift to your sisters, with your husband's assistance, that would be a good deed; for your charity to him is both charity and an act of maintaining ties. But if the land has been designated for the recipient of the gift, then it has become an endowment (waqf), and it is not permissible to revoke an endowment, even if it has not been taken possession of, according to the more preponderant opinion.