What is the ruling on accepting an invitation from companies for a meal as a marketing strategy, knowing that there is no obligation to deal with them, and does the ruling change if the manager is aware of this?

If the invitations are part of the companies' marketing policy and not intended to influence employees, then there is no harm in attending them. Otherwise, they are not permissible and are considered bribery. The knowledge of the director in government institutions does not change the ruling, because he is an employee and the prohibition applies to him.