What is the legal ruling regarding transactions that occur as an intermediary between internet service providers and customers, where the amount is received from the customer in a hard currency and transferred to the providers in local currency, with a fixed reference exchange rate and a profit margin for the intermediary, and sometimes a delay in transferring the amount due to the closure of exchange offices?

The work you describe is an agency for service providers in receiving and transferring funds, which is permissible in principle, and you may take a commission for it. Agency for a known fee is permissible, though scholars differed on whether the commission can be a percentage of the amount, with some allowing it.

If the commission is a percentage, then this work is permissible. However, it must be agreed upon that the actual exchange rate will be used, not a fixed reference rate, because you are an agent for the service providers in the exchange process. You do not guarantee exchange rate fluctuations; any decrease or increase is borne by them or returned to them, unless they authorize you to benefit from the increase. An agent does not guarantee anything except in cases of transgression or negligence.