Is it permissible to take a sum of money or participate in the capital of a contractor who has been nominated to work for a company, knowing that the company owner is the one who agrees upon the prices?

It is not permissible to take payment from contractors unless authorized by the company owner, as long as bringing in contractors is part of your job. Likewise, if you are a partner with the contractor, it is not proper to nominate him for work unless the company owner is aware of your partnership. However, if another employee nominates him, there is no harm if the company owner is unaware of your partnership, provided that your job has no subsequent relation to the contractor's work. Otherwise, the company owner must be informed.