What do you mean you "don't have any actual on paper proof"? I'm not too familiar with American law and such but don't you have a contract that states what your starting salary should be? A promise that's not put into writing and signed by your boss probably isn't worth anything.
To get a bit closer to your question: be friendly, ask if there has been a mistake or if you might have misunderstood the agreement. Only imply they're doing this on purpose if you're willing to leave the company if things go wrong.