Praise be to Allah.
There is nothing wrong with working as an intermediary between service providers and customers. This comes under the heading of brokerage. That is on condition that the service agreed upon is permissible.
The intermediary introduces the customer to someone who is able to provide the service and guarantees that the work will be done, in return for commission that is paid to him, whether he takes it from the customer, from the service provider, or from both, according to the terms of the agreement, because he is providing a benefit to both of them.
Al-Bukhaari (may Allah have mercy on him) said in his Saheeh: Chapter on the fee of a broker: Ibn Sireen, ‘Ata’, Ibraaheem and al-Hasan did not see anything wrong with the fee of a broker.
Ibn ‘Abbaas said: There is nothing wrong with saying: Go and sell this garment, and anything more than such and such an amount is yours.
Ibn Sireen said: If he says: Sell it for such and such, and anything more than that is yours, or is [shared] between me and you, there is nothing wrong with that. The Prophet (blessings and peace of Allah be upon him) said: “The Muslims are bound by their conditions.” End quote.
It says in Fataawa al-Lajnah ad-Daa’imah (13/129): If there is an agreement between the broker, the seller and the buyer that the broker will take from the buyer, from the seller, or from both of them, a certain amount, that is permissible, and the amount is not limited to a certain percentage; rather whatever is agreed upon and accepted by the one who will pay the commission is permissible.
But it should be within the limits that are customary among the people and what will be fair to the broker, in return for his efforts in mediating and striving to complete the transaction between the seller and the buyer, and it should not be detrimental to the interests of the seller or the buyer by increasing it beyond what is reasonable. End quote.
If the service is haraam, such as transporting alcohol, or building maintenance of a riba-based bank, then it is not permissible to act as an intermediary in such cases, because of what that involves of helping in sin. Allah, may He be exalted, says (interpretation of the meaning): “And cooperate in righteousness and piety, but do not cooperate in sin and transgression. And fear Allah; indeed, Allah is severe in penalty” [al-Maa’idah 5:2].
And Allah knows best.