If there is no bidding and this employee is not responsible for looking for offers and prices, there is nothing wrong with you giving him some money or a percentage in return for his help and brokerage. 

Shaykh Ibn Baaz (may Allah have mercy on him) was asked about the ruling on looking for someone to rent a store or apartment in return for payment from the one whose request is met. 

He replied: There is nothing wrong with that; this is a fee and is called commission. But you have to try hard to look for a suitable place that the person wants to rent. If you help him with that and find a suitable place for him, and you help him to come to an agreement on the rent with the landlord, there is nothing wrong with that, in sha Allah, on condition that there is no betrayal or deceit; rather it is done in an honest and truthful manner. If you are honest in looking for what he wants without any deceit or wrongdoing towards him or towards the landlord, then you will be fine, in sha Allah.

End quote from Fataawa ash-Shaykh Ibn Baaz (19/358) 

See also the answer to question no. 45726 

But if the employee is responsible for looking for offers and prices, in that case taking money comes under the heading of bribes and gifts to workers that are haraam, because of the report narrated by al-Bukhaari (7174) and Muslim (1832) from Abu Humayd as-Saa‘idi (may Allah be pleased with him) who said: The Messenger of Allaah (blessings and peace of Allah be upon him) appointed a man from al-Asad who was called Ibn al-Lutbiyyah in charge of the zakaah. When he came he said: This is for you, and this was given to me. The Messenger of Allaah (blessings and peace of Allah be upon him) stood up on the minbar and praised and glorified Allaah, and said: “What is the matter with an agent whom I send, and he says, ‘This is for you and this was given to me’? Why doesn’t he sit in the house of his father or the house of his mother and see if he is given anything or not. By the One in Whose hand is the soul of Muhammad, no one of you gets anything from it (unlawfully), but he will bring it on the Day of Resurrection, carrying it on his shoulders, even if it is a groaning camel, a lowing cow or a bleating sheep.” Then he raised his arms and said: “O Allaah, have I conveyed (the message)?” three times. 

See also the answer to question no. 87864

And Allah knows best.