Is it permissible to purchase an item in installments for 2000 dinars—knowing that its cash price is 1800 dinars—from a seller who is not a bank, and the installments will be paid via a salary deposited in an interest-based bank?

The basic principle is that it is impermissible for a Muslim to deal with interest-based banks, except in cases of necessity or dire need, provided that no Islamic bank is available, and that the transaction is permissible, such as opening a current account or receiving a salary. If the buyer needs to pay installments through an interest-based bank, there is no objection to that due to need.