If they're able to gather the intelligence without a public signal, they wouldn't be wanting a public signal. Any discussion of "should we tell Israel" would be limited to people who knew of the secret subpoena's existence. If Israel already had an asset within that group, they'd just have that person signal them in a much more clandestine manner than a public payment mandated in a signed contract.
Either Israel already knows about the subpoena, in which case the discussion doesn't matter, or they don't, in which case their asset wouldn't be in on the discussion.
Spy on, insert or recruit an asset from the pool of employees who are involved in any "Should we tell Israel?" discussion. That way, even if an answer is "No, don't alert them", the mere existence of the mechanism provides an actionable intelligence signal.