What is the ruling on an employee of a contracting company partnering with a shop owner in the capital of goods that his company buys on credit, knowing that the employee guarantees the company for the repayment of the debt up to a certain limit?

The person is the company's agent for purchasing its needs, and he guarantees what he buys, which is permissible. As for him becoming a partner with the shop owner to sell goods to the company he is an agent for, this is not permissible unless the company authorizes him to purchase from himself, as Ibn Qudamah (may Allah have mercy on him) allowed in Al-Mughni, reasoning that the agent in this case has been authorized to act for himself, so there is no objection as long as the permission is explicitly stated.