Is it permissible to buy a laptop in installments from a store, provided that the installments are deducted from the employee's salary through the bank, and I pay them back to the employee?

There is no harm in installment sales if the contract is free from prohibited conditions, such as increasing the price upon delayed payment. The shop owner is a creditor to your friend, and your friend is a creditor to you, because he authorized you to purchase the device at its price.