It is permissible for the one who is hired or given a contract to do some work, to hire other people to do this work for less pay and to benefit from the difference in payment, unless the agreement was that the first worker should do the work himself or he was chosen specifically for the task, as in the case where someone hires a well known calligrapher or designer.
It says in Kashshaaf al-Qinaa (3/566):
If one is hired to do some work for pay, such as sewing and the like, there is nothing wrong with him hiring someone to do it for a lesser fee. End quote.
Shaykh Ibn Uthaymeen (may Allah have mercy on him) said:
If a person is hired to do some work on the basis of a contract, if it is said to him: We want you to clean this house every day and you will have one hundred riyals per month, then he hires someone else to clean the house every day, in the way described in the original contract, but for fifty riyals, this is permissible, because it is similar to when we say that it is permissible to sublet a property for the remainder of the rental period for a higher rate than one is paying. And this is how people do things today. You will find the state - for example - giving a contract to a company to clean mosques, and for each mosque they pay such and such per month. Then this company brings workers to do the work agreed upon for less than a quarter of what was agreed with the government. But if there is a particular reason for hiring that specific person, then giving the job to someone else is not permissible. For example, if you hired someone to copy out Zaad al-Mustaqni (a book of Hanbali fiqh) for you, and you know that this man has good handwriting or makes few mistakes, then he hired someone who has beautiful handwriting to copy it for a lower price, the scholars say that this is not permissible because what matters is copying properly, not just beautiful handwriting. Rather what matters is beautiful handwriting, proper punctuation and minimum errors. How many people have the most beautiful handwriting but they make too many mistakes. Many students have good handwriting but they do not follow the rules of writing; many people have poor handwriting that no one can read except one who is used to it, but there are no errors in his writing. Whatever the case, the point is that whoever is hired for a skill he has, it is not permissible for him to get someone else to do it for him.
End quote from al-Sharh al-Mumti, 10/39.
Based on that, there is nothing wrong with accepting work and hiring out people to do it for less pay, on condition that the advertisements in question are for halaal things. For more information please see the answers to questions no. 93376 and 7834.
And Allah knows best.
Reproduced from Islam QA