What is the ruling on a father's oath to divorce the mother if she gives him 200,000 riyals?

It appears your father made his wife's divorce contingent on her giving him a sum of money, in the form of a promise. If she does not give him the money, he is not obligated to do anything. If she gives it to him and becomes involved in a hardship because of it, such as selling her belongings, then he is compelled to issue the divorce. If she does not become involved in a hardship and he does not divorce her, he is obligated to offer an expiation for an oath (kaffarat yamin) if he swore by Allah. This is because the default for this phrasing is a promise, not an obligation, unless there is an indicative context or statement, or if hardship occurs.