about job offer letter for EB-1B application

hi!,
I got an offer of a postdoc position at a top University. The title is research associate, the professor said he can be flexible about the title. However he said the position has to be renewed year by year, I wonder if that's what the offer letter is going to say, will it be a problem for applying to Eb-1B? What should the offer letter write in order to be qualified for Eb-1B?

Thank you so much!