Is investing in Islamic banks and earning effortless profits 100% permissible, or is there some doubt in it?

If an Islamic financial institution operates under the strict supervision of a Sharia supervisory board that regulates its transactions to ensure compliance with Islamic law, free from usury and all other prohibited dealings, then there is no objection to investing in it. Any profits generated from such transactions are lawful and permissible.

Moreover, if profits are earned effortlessly through lawful transactions, there is no harm in that. It is well-known that Allah may bestow abundant provision upon a servant, and one may earn substantial wealth from a single deal. For example, if someone buys land at a low price, and its value then multiplies, they sell it and earn significant wealth in a short time with little effort. There is no objection to such a scenario.