Is it permissible for a fasting person who has a debt of stealing to fast three days as an expiation for an oath instead of feeding ten poor people?

The current debt takes precedence over the expiation for an oath. Fasting is sufficient for expiation. There is no difference between a debt resulting from a permissible transaction and a debt from an unlawful right, such as theft, for both are rights of the created being that must be fulfilled.