Does the husband’s saying: "By oath, you will not leave" constitute an oath of divorce, and what is the ruling on him retracting this statement and allowing her to leave?

The ruling on the statement "upon me is the oath" (علي اليمين) depends on the intention of the swearer. If he intended divorce by it, then it is a divorce. Otherwise, the matter is as he intended. If he had no intention, then the custom of his country is applied. So, if the custom dictates divorce, then divorce becomes binding on him according to the custom. If it does not dictate divorce at all, then he must offer expiation for breaking an oath (kaffarat yamin). It is not permissible to retract an oath merely because it has been made, unless the swearer was compelled to make it for a specific reason (the 'busat' — a common colloquialism for being pressured or coaxed), or he intended a specific time for it. In such cases, divorce does not occur if the sworn-upon condition takes place after the time has expired or the reason has ceased to exist.