I'll be a monkey's uncleOften preceded by well: expressing complete surprise or disbelief.Well I’ll be a monkey's uncle! I would never have thought that tourists would go into space!
you scratch my back and I'll scratch yoursIf you do me a favor then I will do you a favor; quid pro quo.Everybody's hustling for a buck and a dime / I'll scratch your back and you scratch mine
I'll beExpressing surprise.Johnny passed his exams despite not heeding your advice to study hard. Well, I'll be!
I'll call the policeIndicates that the speaker is going to call the police, presumably to counter actions from the interlocutor.