What is the ruling on someone who swore by Allah to abstain from eating sausages, then ate them based on a fatwa he heard, and does this entail sin or an expiation of fasting three days?

Swearing off something permissible is allowed, but if you consume it, you must offer expiation for breaking an oath. This is based on the hadith from Abu Hurayrah: "Whoever takes an oath and then sees that something else is better than it, let him do that [which is better] and offer expiation for his oath." The expiation for an oath is feeding ten needy persons, or clothing them, or freeing a slave. If you cannot find any of these, then fasting for three days. There is no sin in swearing off a particular food.