Question: Is it permissible for a person who has qada (omitted) salats to perform nafila (voluntary) salats? What are the major voluntary salats?
ANSWER
[To carry out a salat within its prescribed time period is called adaa. To carry it out after its prescribed time has ended is called qada. Throughout this article, we will use the verb make up to mean to do qada of a salat.]

Salat is missed in two ways:

1. One misses it due to a Shar'i reason, such as sleeping and forgetting. This salat is called faita, that is, a missed salat. If one has missed salat due to such an excuse, one is allowed to perform voluntary salats before making it up because it is written in such valuable books as Radd-ul-Mukhtar, Halabi, Tahtawi, and Fatawa-i Hindiyya:

"It is necessary to make up faita salats [that is, those salats missed due to an excuse] at once. Though it is better to make up faita salats than to perform voluntary salats, to perform pre-fard and post-fard sunnat salats and voluntary salats praised in hadith-i sharifs, such as Salat ad-Duha, Salat at-Tasbih, Tahiyyat-ul-Masjid, Salat al-Awwabin, is better than to perform a salat missed due to an excuse."

The reason why it is better to perform voluntary salats in this case is that it is not a sin to miss a salat due to a Shar'i reason, such as forgetting, sleeping, etc. It is not a sinful act for a person in this situation to delay his/her qada salats as much as to perform the abovementioned voluntary salats. The total number of salats missed due to forgetting or sleeping can be few. For example, if one oversleeps and misses Salat al-Fajr, one must make it up 50 minutes after sunrise. It is permissible for one to perform Salat ad-Duha before making it up because one is not sinful for having missed Salat al-Fajr due to oversleeping, so one does not incur a sin if one delays making it up as much as to perform Salat ad-Duha.

2. One omits salat deliberately out of laziness, which is a major sin. Do not confuse a salat not performed deliberately with a salat missed due to an excuse. The former is a major sin while the latter is not a sin. This major sin becomes twice itself in wickedness at each length of time wherein one could make it up spent at leisure, for it is fard (obligatory) to make up a missed salat immediately at leisure.

Hadrat Abd al-Qadir al-Ghaylani states:
It is idiocy to spend time performing sunnat salats while one has salats to make up because the sunnat salats of a person who has salats omitted are not acceptable. A person performing the sunnats while he/she has debts of omitted salats is like a debtor taking a present to the person to whom he/she owes, which, normally, is not acceptable. A Believer is like a business person. The fard salats are his/her capital, and voluntary salats are his/her profit. Unless the capital is rescued, they will give no profit. It is stated in a hadith-i sharif:
(If a person who has a salat to make up performs a voluntary salat before making it up, he has strived in vain.) [Futuh-ul-Ghayb]

Abdulhaqq-i Dahlawi, a scholar of the Hanafi Madhhab, stated, "This hadith indicates that none of the voluntary salats of those who have salats omitted will be accepted."

Hadrat Imam-i Rabbani states:
"Voluntary acts of worship are of no value when compared with obligatory ones. The value of voluntary acts of worship compared with obligatory ones is not even like that of a drop of water compared with an ocean" (Maktubat, Letter 260).

"Those who have salats to make up must perform qada salats at the times when it is praised to perform voluntary salats. By doing so, they will have both made up their missed salats and will have earned the reward of voluntary salats. When performing Salat ad-Duha, Salat al-Awwabin, Salat at-Tahajjud, or Tayiyyat-ul-Masjid, one should intend, for example, "Of all the Fajr salats that I must do qada of, I intend doing qada of the first Fajr salat and I also intend performing Salat ad-Duha." Thus, one will have both made up one's missed salats and will have performed aforesaid voluntary salats" (An-Nawadir-ul-Fiqhiyya fi Madhhab-il-Aimmat-il Hanafiyya).

"Even if one does not have any salats to make up, it is still permissible for one to perform qada salats" (Radd-ul-Mukhtar, Fatawa-i Hindiyya).