Is it a condition for a woman to seek her husband's permission before fasting voluntary fasts?

And does her fast become invalid if he orders her to break it and she does not agree?

Yes, regarding a woman's voluntary fast, the Prophet ﷺ said: She should not fast except with his permission, meaning voluntary fasts, she should not fast except with his permission, and permission is of two types.

Either it is silent approval, he knows she is fasting and remains silent, this is permission.

For example, he knows she is fasting, he knows she fasts every Monday or every Thursday.

Or she fasts Mondays and Thursdays, or the white days, he knows about it and is silent. This is permission, this is permission, she does not have to explicitly ask: Do you agree if I fast tomorrow? No, he has already permitted, he has no problem.

But if he tells her to break her fast, she should obey him, because obedience to the husband is obligatory and fasting is voluntary, just like when she prays.

If a woman is praying a voluntary prayer and her husband calls her, there is nothing wrong with her stopping the prayer.

And the son as well, if his mother calls him, he should stop the prayer if it is voluntary. The story of Jurayj the worshipper is very well known.

When he said, "My mother or my prayer, my mother or my prayer," Allah punished him because he did not stop his prayer.

And he should answer his mother. So the mother and father have an obligatory right that comes before voluntary prayer.

And likewise, the wife has an obligatory right that comes before fasting.