I used familyfunds lastname -10000 a little while ago in my game. It's sometimes tricky to do. It worked for me, after a few tries.
Let me see if I can re-create the issue and see if I can find a fix.
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Edit to Add:
To get the cheat to work it needs to have testingcheatsenabled on.
familyfunds lastname amount
Example:
familyfunds Haskell-Chandler 100000
If it doesn't work, try looking at the household's last name by looking at their lot in Edit Town mode. As the cheat uses the household's last name.
I was having problems with it in my test-game. I'm telling you, it's tricky. But I got it to work by going by the household's last name. Not by my main Sim's last name. If that makes any sense.
