Is currency trading unlawful? Tell me specially about digital currencies e. g crypto, bitcoin, etc

It is lawful. You can trade in currencies, GBP for USD, USD for Euro …etc. You can also sell currencies with difference in cash transactions only. For example, you sell a note of 100 GBP for 5notes of 20 GBP, for 102 GBP.

However, as for electronic currencies, there is no enough information, therefore S. Sistani is not giving any fatwa