I support it, but i would also like to add some things

The spy will be able to find out it the Lords treasury is wherever he is infiltrating, should be easy enough askng around in taverns or the right people. May raise suspicion but its never cause for death, in theory^^ He should also be able to get an idea of how rich the treasury is. (useful to know regardless)

The amount of money stolen should be a percentage of the total treasury, with a minimum amount and perhaps maximum. lets say 750+20% out of total.
Large treasury its too much to carry, small treasury and he grabs the box with jewels :)

The infiltrate action should cost the agent his subterfuge action for the turn, imo he should be able to attempt to steal the same turn he infiltrates.

I dont like the idea of stealing income from cities, only lords treasuries :)