Nothing. Tell them they can put in the extra 150K themselves if they want to keep the others out. They should have 1st rights if they did it correctly.
this. do not complicate your cap table early with so little capital raised, think long-term. raise the 150K from value-add angels, emphasize "making the pie bigger" to existing investors. Startups and startup investing is unusually binary in outcome: you should focus on increasing your odds of success rather than maximizing value in case of success.