Question: While one has missed zakat, Ramadan fasts, other obligatory acts of worship to make up, is it permissible to do voluntary acts of worship of the same kind?
It is not permissible because an obligatory act of worship is highly important. It is stated in hadith-i sharifs:
(The most meritorious jihad is to do obligatory acts of worship.) [Imam-i Ahmad]
(While everybody is occupied with voluntary acts of worship, do obligatory ones!) [Miftah-un-najat]
(Nothing brings you closer to Allahu ta’ala better than doing obligatory acts of worship.) [Bayhaqi]
(He who does obligatory acts of worship will be the most devout [abid] of humans.) [Ibn Adiy]
(Voluntary salats of a person are not accepted if he has missed salats to make up.) [Durrat-ul-fakhira]
In his will to Hadrat Umar, Hadrat Abu Bakr said:
If you do a duty in the morning that Allahu ta’ala obligated you to do at night or vice versa, He does not accept it. He does not accept your voluntary acts of worship unless you do obligatory duties made fard on you. [Kitab-ul Kharaj]
Hadrat Imam-i Rabbani stated:
The value of voluntary acts of worship compared with that of obligatory ones is not even like a drop of water compared with an ocean. [Letter 260]
If a voluntary act of worship causes one to omit what is obligatory, then it is not considered an act of worship; it will be harmful. [Letter 123]
In conclusion, if one has zakat debt of 1 Turkish lira and without paying it if one gives money for the construction of 100 mosques and donates billions to billions of humans, it will not be accepted.