Change it to a PC instead and adjust the amount on the Journal tab. Then change it back to a NPC.
Also why not just give the pocket change and the WBL value just once together instead of trying to give different amounts of starting value. Your trying to use the Starting Value as an adjuster instead of the "starting value".
I am setting the initial amount to be what they should have as an NPC of their level, then buying equipment, and then modifying the starting value to reduce what is left the the amount of pocket change.
Now, the reason I'm doing it this way is because I am converting NPC's from 3.5. NPC gear value (and the value of some items) have changed between 3.5 and Pathfinder, so to be sure that the gear they're carrying falls within the amount allowed by the new Pathfinder guidelines (and item costs), I have to buy their equipment using those WBL guidelines. Then, after I'm done purchasing their gear, they normally within the module's stat blocks only have, say, 33 gp, but have a little over 500 of their NPC gear value left. Let's say 500 for a good round number.
500 - 33 = 467
Then, if their starting wealth was, say, 2,400, I'd go into the starting wealth and change it from 2,400 to 1,943. This SHOULD leave the NPC with 33 gold, but instead leaves him with -33 gold.
Is this the simplest way to change the current wealth of an NPC? Probably not, but it doesn't matter, the fact is that something is causing the arithmetic in the program to not calculate wealth correctly when starting values get changed. That's a bug, and it's a problem for me, the customer.