What is to be done by someone who vowed to donate all their savings as charity, then circumstances forced them to withdraw from it with the intention of returning it? And are they sinful for that, and does it entail expiation?

Since you swore to donate the saved money for the sake of Allah, and then spent some of it on yourself, you have not fulfilled your oath. Therefore, an expiation for breaking an oath (kaffarat yamin) is incumbent upon you. The expiation is to feed ten poor persons, or clothe them, or free a slave. And whoever cannot find [the means], then a fast of three days. Your oath has been dissolved by the breaking of it, and you may now benefit from the money or donate it.