Is it permissible to purchase a bank guarantee in exchange for a bank portfolio?

If the intent is to issue a bank guarantee based on a portfolio held by the issuer, there is no objection to it, whether the value of the portfolio is less than, equal to, or greater than the guarantee amount. A covered letter of guarantee is considered an agency (wakala), and there has been a detailed exposition on the ruling concerning the permissibility of charging a fee for it.