I should've mentioned it's an European entity, a prime broker named Ex Ante Limited.
Perhaps put in US terms, a customer grants a lien to the deposit for the Prime Broker. It seems similar to these terms from Interactive Brokers Clearing Agreement:
I wonder if there are any futures brokers...
Would you sign to these terms with a prime broker (PM) or FCM? They are meant for a "professional" client:
The wording doesn't look too good, but I suspect that such terms are the industry standard and there is no better choice if trading futures.