Is it permissible to accept a job offer that includes a penalty clause for taking clients, and an interest-based clause for delayed invoice payments, knowing that the penalty clause is impossible to enforce?

If the agreed-upon work is permissible, then there is no objection to accepting it. The stipulated conditions—such as not dealing with the company's customers in the future, or paying a percentage of purchase invoices upon leaving the company, or imposing an additional amount for late payment—do not affect its permissibility. These conditions are not legally binding and do not constitute usury (riba).