Is what the sales manager does—transferring some work to other companies for a fee, or undertaking it for his own account and benefiting from the price difference, while ensuring that this does not conflict with the primary interests of the main company—permissible or forbidden?

If your company cannot perform certain tasks, there is no harm in directing those who seek these tasks to someone who can perform them. However, taking money in return involves two prohibitions: 1. It falls under the category of prohibited gifts to employees, unless with the employer's permission, based on the hadith of Ibn al-Lutbiyyah. 2. You are a special employee, and your time belongs to the employer. Therefore, it is not permissible for you to undertake private work during working hours except with the employer's permission. Exceptions are made for what is permitted by Sharia, custom, or agreement.

As for undertaking these tasks for your private account outside the company, the second prohibition applies if the agreement is made during working hours; otherwise, there is no harm in it.