Sounds like an incredibly uncouth act to me. Someone offers you employment (which is a huge plus), wants to help you get an IR (a huge plus) and asks only that you work for them for x amount of time...and before you even start you know you're going to shortchange them?
Don't get me wrong, I'm not saying all employers would be fun to work for, but if you know starting out that it's not the place for you perhaps you should keep looking. And not just in this industry, but in life in general, it pays to maintain your integrity and keep a solid reputation.