You can add a default stub for the fire! method which will call original implementation:
boss.stub(:fire!).and_call_original
boss.stub(:fire!).with(employee1).and_return(true)
Rspec 3 Syntax (@pk-nb)
allow(boss).to receive(:fire!).and_call_original
allow(boss).to receive(:fire!).with(employee1).and_return(true)