What is the ruling on a woman who underwent a tubal ligation without knowing it was forbidden; does she owe expiation?

Birth control is of two types: 1. Permanent birth control: This is not permissible unless pregnancy poses a danger to the mother's life or causes her unbearable hardship, and it does not include what is common among women. 2. Temporary birth control: This is permissible to allow the mother rest between pregnancies or for raising children.

As for preventing or delaying pregnancy out of fear of poverty, this is not permissible, because Allah has guaranteed provisions.

If the ligation is permanent and permissible for you, then there is no blame. Otherwise, you must repent and remove the ligation if possible. If the ligation is temporary, it must be removed once the excuse for it is no longer present.