Assalam alaikum warahmatullahi wa barakatuhu,
الجواب وبالله التوفيق
As per the situation described in your question if you break the oath and pay the kaffarah for breaking oath, you will be free from the binding condition of the oath. In order to refrain from sins you should use another binding condition which you have to fulfill but without taking an oath.
In the current situation the amount originally set as part of the oath cannot be reduced or replaced by nafl salah etc. As long as the oath is in place one will have to fulfill the binding conditions of the oath and pay the amount they included as part of the oath.
واللہ اعلم بالصواب