Yes I'm sure you can take up the promotion. I think the HR doesn't want you to get that pay rise so they are making up bullshit excuses. As long as your not changing your profession and your with the same employer this should not be a problem at all. A simple fix to this is - according to the 187 visa requirements, you cannot quit without substantial reasons but it's much easier for the employer to dismiss you. So ask your boss to fire you on Friday and rehire you as a sous chef on Monday with a new contract.