Is it permissible to use bank guarantees for a specified percentage of their value and for a limited period?

The gist of the answer: It is impermissible to borrow with a reduced interest rate because any increase in the loan or benefit gained by the lender constitutes riba (usury/interest). There is no difference whether the borrowing is direct or through an agent. Clarification is requested regarding the nature of "guarantees" to determine their Sharia ruling.