Is an oath suspended on committing a sin, and which includes an expiation difficult to fulfill, valid? And can one be absolved from it and offer expiation for it?

If a person swears to commit a sin and then breaks the oath, he must pay an expiation for the oath (kaffarat yamin). If he swears that he will be an unbeliever (kafir) if he commits a certain sin, and then he does not fulfill what he committed to, he does not become an unbeliever. However, he must pay an expiation for the oath, because scholars have agreed that whoever swears in such a manner does not become an unbeliever; rather, he must pay an expiation for the oath. In summary, two expiations are required: an expiation for swearing by Allah, and an expiation for swearing to be an unbeliever.