Tafsir Ibn Kathir -> Surah At-Taubah -> The Riqab

Al-Hasan Al-Basri, Muqatil bin Hayyan, `Umar bin `Abdul-`Aziz, Sa`id bin Jubayr, An-Nakha`i, Az-Zuhri and Ibn Zayd said Riqab means those slaves who make an agreement with the master to pay a certain ransom for their freedom.'' Similar was reported from Abu Musa Al-Ash`ari. Ibn `Abbas and Al-Hasan said, "It is allowed to use Zakah funds to buy the freedom of slaves,'' indicating that `Riqab' has more general meanings than merely giving money to slaves to buy their freedom or one's buying a slave and freeing him on an individual basis. A Hadith states that for every limb ﴿of the servant﴾ freed, Allah frees a limb of the one who freed him from slavery, even a sexual organ for a sexual organ, for the reward is equitable to the deed,

﴿وَمَا تُجْزَوْنَ إِلاَّ مَا كُنتُمْ تَعْمَلُونَ ﴾

(And you will be requited nothing except for what you used to do.) ﴿37:39﴾