Is dealing with Al Baraka Bank to purchase a device by paying a cash amount or a check to the seller, with authorization from the bank and repayment in fixed installments with a 6% increase over the original price, considered permissible (halal) or does it fall under usurious transactions (riba), and what is the legitimate alternative if it is not?

If the bank authorizes you to purchase the device, and then owns it and sells it to you, there is no harm. However, if you undertake the entire purchase process and take possession of the item for yourself, it is not a real sale but rather a deception and usury. The legitimate alternative is for a bank representative to purchase the item, own it, and then sell it to the client. It is permissible to appoint the client himself as an agent when necessary, provided this condition is met, or for the bank to have subsidiary companies that sell these goods.