Sims bill higher than 9,999,999 simoleons

Are you using the ATM mod ? That is usually the culprit for this bills. You should remove it

I don't think the bill will go back down so try if you can get rid of it by moving (not sure if it does)

If not, try this (don't know if it works though, so make sure to have a backup): Add a random Sim you don't care about to your household. Then move all the other Sims out to the not in world bin. Go to map view, evict the Sim you don't care about and delete him. Then move the rest of the family back in.