What is the ruling on an employee taking the difference between the price he pays to stores from his own money and what is recorded on the invoices he submits to the company, knowing that he agrees with store owners to reduce the amount in exchange for immediate payment without pressure? And how can this process be regulated to be permissible (halal)?

Shop owners sell their debts to the inquirer for less than their value in cash. This is impermissible as it involves riba al-nasi’a (usury of delay) and riba al-fadl (usury of surplus). The inquirer must repent and return the usurious amounts to their owners. There is no permissible way for this transaction to occur unless the inquirer acts as an agent for the shop owners in collecting their debts for a specified fee.