What is the ruling on a husband forcing his wife to swear an oath of not cheating, while the wife fears sin despite her truthfulness in the oath?

We understand from your question that you swore to your husband that you did not betray him, and that if you did betray him, he would leave you. If no betrayal occurred from your side, then there is no sin upon you because you were truthful in your oath. As for his statement "he will leave you," it is a promise of divorce that does not take effect. We caution against the danger of a husband suspecting his wife without evidence, and the wife should adhere to the manners of Islamic law and avoid situations of doubt.