The basic principle is that the One who is to be described as wali an-ni‘mah (benefactor or source of blessings) is Allah, may He be exalted, Who bestows His abundant blessings, both visible and invisible, upon His slave.
Ibn al-Qayyim (may Allah have mercy on him) said:
All goodness is to be attributed to Allah; it is in His hands, by His leave and from Him. He is the benefactor and source of the individual’s blessings, as He is the One Who initiated them without anyone being entitled to them; He bestows them upon him, even though the individual may be displeasing Him by his turning away from Him and his negligence and sin. So Allah deserves all praise and thanks, and the slave deserves blame, criticism and shame.
End quote from al-Fawaa’id, p. 113.
But that does not prevent any of His slaves whom He has blessed from being a benefactor or source of blessings to another of His slaves. Yet it must be noted that there is a huge difference between the true blessings of Allah to all of His slaves, as He is the Creator of those blessings and the One Who divides provision among them and causes provision to come down from His stores, and the blessings that some of His slaves bestow upon others, from what Allah has given to them and caused them to possess and put under their control. They are no more than a means of directing the blessings of Allah to other slaves of Allah. The blessings bestowed by the Creator are unlimited, whereas the blessings bestowed by people are limited to what Allah has given to them.
Giving the name “benefactor” or“source of blessings” (wali an-ni‘mah) to the one who does a favour is something that is known in Arabic language and in Islam. The closest thing to that and the most well-known example is the use of this name for a master who has manumitted a slave.
Al-Bayhaqi (21966) narrated that Huzayl ibn Shurahbeel said: A man came to ‘Abdullah ibn Mas‘ood and said: I manumitted a slave of mine and made him a saa’ibah (a freed slave with no wala’ connection to anyone), then he died and left behind some wealth. ‘Abdullah said: The people of Islam did not free slaves as saa’ibah; rather the people of the Jaahiliyyah used to do that. You are his heir and his benefactor or the source of his blessings (wali ni‘matihi). If you are not comfortable with that, then show it to us and we will put it in the bayt al-maal (the treasury of the Muslims).”
This report was originally narrated by al-Bukhaari, 6753
Al-Qaadi ‘Iyaad (may Allah have mercy on him) said in al-Mashaariq (2/18):
The “benefactor” or “source of blessing” (wali an-ni‘mah) is the one who manumits a slave. End quote.
Al-Jassaas (may Allah have mercy on him) said in Ahkaam al-Qur’aan (2/231):
(It refers to) the master who has manumitted a slave, because he is the source of the favour of manumission. Hence he is called “source of blessing” or “benefactor” (wali an-ni‘mah). End quote.
He also (may Allah have mercy on him) said:
He (the Prophet (blessings and peace of Allah be upon him)) made the rights of the “benefactor” or “source of blessing” (wali an-ni mah, i.e., the master who manumits a slave) like the rights of the father. The evidence for that is the hadeeth: “No son can repay his father unless he finds him enslaved and buys him and manumits him.” (Narrated by Muslim, 1510).So he described the ransom of the father as equivalent to his rights (over his son), and equal to his favours to his son.
End quote from Ahkaam al-Qur’aan, 1/169
See also: Sharh Muntaha al-Iraadaat, 2/500; Kashshaaf al-Qinaa‘, 4/405; Ikhtilaaf al-A’immah al-‘Ulama’, 2/85; Anees al-Fuqaha’, p. 98; al-Fawaakih ad-Dawaani, 2/250
In linguistic terms:
Ibn Manzoor (may Allah have mercy on him) said:
The master who is the “benefactor” or “source of blessings” is the one who manumits the slave i.e., he blesses his slave by manumitting him. End quote from Lisaan al-‘Arab, 15/405
See also: Tahdheeb al-Lughah, 5/205; al-Misbaah al-Muneer, 2/614; Taaj al-‘Uroos, 40/243.
Based on that, there seems to be no reason not to use this phrase to refer to some people, bearing in mind the difference mentioned above. However there is the fear that this matter may involve some going to extremes and exaggerating about people. In that case it should not be used for that reason, not because a person cannot be a source of blessing to another.
And Allah knows best.