What is the ruling on a partnership in capital and technical consultations with a subcontractor who works with the contracting company I am employed by, without the company's knowledge, given that this does not affect the work or involve any favoritism?

Your partnership with the contractor is permissible in principle. However, it is not permissible for you to work in your company during your official working hours without permission. Likewise, it is not permissible for you to supervise him without your employer's knowledge that you are his partner, because you are their representative, and your supervision of your partner's work is like supervising your own work. This contradicts fairness to your company and thoroughness for your employer. Therefore, either you inform your employer and they grant you permission, or someone else takes over the supervision. As for your work with your partner outside official working hours, in a way that does not affect your primary work, there is no harm in it.