In the name of God
This is not giving commission. The term "bribery" better defines this action. In such contracts, the only thing which must be noted is to consider whether the company deserves to be given the contract or not. So, it is not permissible for the employees to get such money for the job they have been hired to do and for which they receive a salary. Nor is it permissible for the customers to entice the employees by tipping them to finish their business because this practice is bound to spread corruption.
Allamah Sayyid Muhammad Husayn at-Tabataba'i writes in his Islamic Teachings in Brief:
Receiving money or a gift in return for a judgment or for the accomplishment of a task which is the duty of the receiver is called
Bribery is a mortal sin in Islam. The person who commits this sin is
deprived of many social privileges of religion (justice) and deserves
Allah's punishment. This matter has been specified in the Holy Book
and the ahadith.
The Holy Prophet (S) has cursed tile givers, receivers, and the
mediators of bribe.
The sixth infallible Imam (as) also states: "Receiving bribe for
administering justice equals blasphemy to Allah".
Surely, all these reproachments apply to the bribe taken for the
judgment of truth or for administering justice. The sin of the bribe
taken for the judgment contrary to truth and for unjust deeds is far
greater and its retribution is much severer.
Bribery is allowed only in the case that you want to get to your right (in a way that other's right is not ignored).