Conclusion: If you need a bank guarantee or letter of credit, let it be issued through an Islamic bank, and it is more appropriate for you to have money in the bank to cover the liability. The bank fee should be in return for acting as an agent (wakaalah) and not as a guarantor (kafaalah), so the fee should not be connected to the amount of liability or the length of the guarantee period. If you cannot find an Islamic bank, then there is nothing wrong with requesting this guarantee from a riba-based bank, if you deposit enough money to cover any liability in a current account, and the bank fee has nothing to do with the amount of liability or the length of the guarantee period Dealing with riba-based banks, without getting involved in riba, is permissible, as is depositing money in them in the case of need, when there is no Islamic bank. As for working in such banks, that is prohibited, even if it is working as a security guard, because that is helping, directly or indirectly, with riba. And Allah knows best.

There is nothing wrong with opening current accounts - without interest - in riba-based banks, if there is a need for that and there are no Islamic banks that could meet the customer's needs without much trouble and difficulty.