Is money earned from freelance work, agreed upon with a colleague, then assigned to the office where the person works, with him receiving a percentage of the profits without the employer's knowledge, lawful or unlawful?

If bringing business to the office is part of your job requirements, then it is not permissible for you to take a commission on it and conceal this from your employer. This is considered a betrayal, and taking the money would be unlawful. You must be open with your employer. If he permits it, then it is permissible; otherwise, it is not permissible to take anything for yourself without his knowledge and consent. Even if you are a partner, trustworthiness and honesty are obligatory, because injustice and betrayal destroy blessings and end partnerships. Allah Almighty says: "I am the third of the two partners, as long as one of them does not betray his companion. If he betrays him, I depart from between them."