Am I sinful for not sending gifts (clothes) to a former fiancé after breaking off the engagement due to his lying and uttering the oath "by God, it is Haram upon my religion" (علي الحرام من ديني), given that I suffer from diabetes and am looking for a husband who fears Allah and observes his prayers?

You are not wrong for breaking off the engagement, as a woman is permitted to withdraw from it even without a reason. As for the gifts, if they were part of the mahr (dowry), you must return them to him since the marriage was not completed. If they were merely a gift, then according to the Maliki school, the predominant view is that they must be returned if the withdrawal was initiated by you. Try to return them to him through his relatives or acquaintances without direct contact. If that is difficult, then give them away as charity.