Is investing money in associations that sell electronic devices through an Islamic Murabaha system, with seemingly fixed monthly profits, considered permissible despite the absence of a specified profit percentage in the the contract?

This transaction is a Mudarabah (definition) (profit-sharing partnership), and the fundamental principle in it is to agree on a known percentage of the profit to prevent ambiguity and disputes. If the two parties are silent about the profit percentage, then recourse is made to custom (urf). If no custom exists, the Mudarabah becomes void, and the Mudarib (working partner) receives the wage of the like (ajr al-mithl). Since the profits are semi-fixed (15,0 dinars per million), and this is the usual practice, there is no objection to it as long as the company is not obligated to a specific amount and the matter is subject to profit and loss.