That and tax evasion is a serious crime.
The easiest means to pay extra monies off the pitch would be is to insert appearance-type clauses in contracts. Don't know if those count towards FFP or other league rules towards compensation against revenue. But they can easily offer a million here or there for a player to show up in UAE for an event, or a charity cause, or a friendly match of sorts, or whatever they desire.
If any compensation spelled out in a contract does go against FFP, they could offer off the books through club internal communications. Tell a player like Haaland to show up in a summer/winter event and collect your appearance from the organizer (who is likely linked directly to ADUG). Or, hey, we'll sign you to a separate endorsement/sponsorship with a company in UAE which you'll make a singular appearance annually.