Praise be to Allaah.
You should not observe voluntary fasts when
you still owe one or more days from Ramadaan, rather you should start with the fasts that you owe from Ramadaan, and then observe voluntary
If you fast the tenth and eleventh of Muharram with the intention of making up days that you
owe from Ramadaan, that is permissible and will make up for two of the days that you owe. The Prophet (peace and blessings of Allaah be upon
him) said, “Actions are judged but by intentions, and every person will have but that which he intended.”
Fataawa al-Lajnah al-Daa’imah,
We hope that you will have the reward for
making up the missed fast and the reward for fasting that day.
Fataawa Manaar al-Islam,
by Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him), 2/358.