Is it permissible for someone who is observing a compensatory fast (qada') to change his intention to a supererogatory (nafl) fast so that he can break it due to urgent circumstances, knowing that breaking an obligatory fast is forbidden?

The Shari'ah prohibits stratagems and has warned those who commit them. Therefore, whoever begins an obligatory act of worship, such as fasting, is not permitted to break it except for a legitimate excuse, such as illness, based on the Almighty's saying: "And do not invalidate your deeds." If there is a legitimate excuse, one should break the fast, and there is no need to turn it into a supererogatory act with the intention of invalidating it.