Rewarding Lord Theony
The threat of being audited by Alexander was enough for most nobles to give the correct amount of tithe to him.
Too many nobles had too many skeletons in their closets that they preferred to keep hidden and did not want Alexander to go rummaging through them.
And besides, who knew if Alexander would swindle them of much more than half by underreporting the value of some product?
As for hiding the contents of the treasury somewhere else, well a big pile of gold and other precious metals and stones was surprisingly hard to hide.
And more than that, the ire they would have to face if they were found out made it not worth the trouble.
So the nobles bit the bullet and paid up, believing what Alexander promised would indeed come true.