What is the ruling regarding a woman who intended to donate her gold for the sake of Allah, but then was compelled to sell it? And is it permissible for her to intend to donate the value of the gold, with an additional amount, at a later time?

The general principle regarding donations, such as gifts, presents, and charity (ṣadaqah), is that they are not finalized until they are taken possession of (qabḍ). The proof for this is what Malik narrated in the Muwaṭṭa' from Abu Bakr and Umar, may Allah be pleased with them both. Therefore, any such donation that has not been taken possession of allows the donor to retract it. Accordingly, there is no blame upon you for retracting this charity, and you bear no sin for doing so, nor are you required to give another charity.