What is the ruling on working for an online hotel room booking company that uses a volatile digital currency and offers a "stacking" feature guaranteeing a fixed monthly percentage of this currency, knowing that its dollar value is unstable? Is it permissible to continue working with them to pay off a debt owed to the company?

It is not permissible for a client to leave a sum of digital currency to receive a percentage of it, because profit as a percentage of capital makes the investment a forbidden usurious loan. The field of investment must be permissible, the capital not guaranteed, and the profit a known percentage of the gains, not of the capital.

As for working to help others book hotels, it is permissible, provided that there is no indication or assistance to usurious investment in any way. Rather, its prohibition must be stated or silence observed, as Allah Almighty says: ﴿And cooperate in righteousness and piety, but do not cooperate in sin and aggression﴾. And the hadith (definition) that cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses. So, assisting in falsehood is forbidden.