What is the ruling on money received by an employee from a party that his institution contracted with, due to his efforts in concluding the contract, given that this money was not agreed upon beforehand, and he considered the institution's interest in his choice?

A gift for work is permissible if it is an appreciation for your efforts. However, if it is from an institution you collaborate with for the benefit of your work, then it is among the forbidden gifts to employees, and it is not permissible to benefit from it except with the permission of your employer. This is because it is given to you due to your position, and it might be offered in hopes of gaining your favor in the future. The Sharia has prohibited such gifts to block the means (to corruption).