Praise be to Allaah.
The contract described in
the question is a contract that is based on reward and there is nothing
wrong with it from a shar’i point of view, so long as the ads are free of
haraam content and do not help with haraam things.
Among the permissible forms
of reward-based contracts that the fuqaha’ have mentioned is when someone
says: “Whoever brings back my stray camel will have such and such”. The
evidence that reward-based contracts are permissible is the verse in which
Allaah says (interpretation of the meaning):
“They said: ‘We have
lost the (golden) bowl of the king and for him who produces it is (the
reward of) a camel load; and I will be bound by it’”
[Yoosuf 12:72]
There is scholarly
consensus that it is permissible. This was narrated by more than one of the
scholars, including Ibn Qudaamah in al-Mughni (6/20), where he said:
We do not know of any difference of opinion concerning that.
A reward-based contract is
similar to a hiring contract, but the rules are more relaxed, therefore a
reward-based contract is valid even though the type of work in question is
not known, as in the case of returning a lost camel: it may be returned
after a lot of work or a little.
Another example is the case
asked about here: You do not know how many users will click on those ads.
So long as the financial
reward is stipulated in return for each user clicking on the ads, there is
nothing wrong with it from a shar’i point of view.
But the content of these
ads should be known, so that they will not be ads for haraam things; in that
case it is haraam to advertise them or to take any payment for that, because
Allaah says (interpretation of the meaning):
“… do not help one
another in sin and transgression”
[al-Maa’idah 5:2]
For more information please
see the answer to question no.
101806.
And Allaah knows best.