Is it permissible to take a sum of money from an employee, as agreed upon in the contract concluded between the two parties, in the event of him suddenly leaving work or not completing the period agreed upon in the contract?

What is known as a penalty clause (or liquidated damages clause) varies in ruling depending on its condition. It is permissible to stipulate it in contracts where the obligation is not a debt, as in the case mentioned in the question. However, in contracts where the original obligation is a debt, it is not permissible.