Praise be to Allaah.
It seems that there is nothing wrong with doing that. Allaah has enjoined certain actions upon His slaves, and has promised a great reward in this world for doing them, so as to encourage people to do them. Allaah says (interpretation of the meaning):
“And whosoever fears Allaah and keeps his duty to Him, He will make a way for him to get out (from every difficulty).And He will provide him from (sources) he never could imagine”
The Prophet (peace and blessings of Allaah be upon him) said: “Whoever would like his provision in this world to be increased and his life span to be extended, let him uphold the ties of kinship.” Narrated by al-Bukhaari, 5986; Muslim, 2557.
As a way of encouraging righteous deeds, the Prophet (peace and blessings of Allaah be upon him) gave permission for the one who killed a kaafir on the battelefield to take his spoils.
It was narrated that Abu Qutaadah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said, during the year of Hunayn: “Whoever kills an enemy and has proof of that will have his spoils.”
Narrated by al-Bukhaari, 2973; Muslim, 1751.
“Spoils” here refers to the money, luggage, clothing and weapons that a fighter has with him.
And the scholars regarded it as permissible to offer prizes for memorizing soorahs from the Qur’aan or ahaadeeth, or for winning a contest of knowledge.
The scholars of the Standing Committee were asked:
What is the ruling on receiving prizes for Qur’aan memorization contests?
There is nothing wrong with that, and there is no difference between men and women in this regard.
Fataawa al-Lajnah al-Daa’imah, 4/126
This has to do with giving and offering: it is permissible to offer and give money to one who lets his beard grow or does other things required by sharee’ah.
But with regard to the one who takes that money: if he let his beard grow in order to take the prize, then he will not be rewarded for his action, but if the prize motivated him to fulfil the command of Allaah, or if he started because of the prize and then changed his intention after that and adhered to (following the ruling), then he will be rewarded for that in which his intention was sound, and it will not matter that at first he was doing it for the prize.
It was narrated from Anas that a man asked the Prophet (peace and blessings of Allaah be upon him) for sheep between two mountains and he gave them to him, then he went to his people and said, “O my people, become Muslims, for by Allaah Muhammad gives generously and he does not fear poverty.”
Anas said: People would become Muslim only for worldly gains, but as soon as they became Muslim Islam became dearer to them than this world and everything in it.
Narrated by Muslim, 2312.
This is how it appears in most copies: “fa ma yuslim (as soon as [they] became Muslim).” In some copies it says “fa ma yumsi (before the day ended).” Both are correct. The first means that shortly after they became Muslim, Islam became dearer to them, i.e., at first they professed Islam for worldly purposes, with no sound intention in their hearts, then by the blessing of the Prophet (peace and blessings of Allaah be upon him) and the light of Islam, their hearts were soon opened to true faith which took root in their hearts, and then Islam became dearer to them than this world and everything in it.
Sharh Muslim, 15/72, 73.And Allaah knows best.