a permanent postion means that

is set by a school permanently, which is possible and reasonable for an institute. Even if you don't take that position, someone else will fill in . So you boss just need to say this position is permanent in our school. he need not mention clearly that this position is just permanent for you. Actually that is all INS requires. But sometimes lawyers and applicants misunderstand this meaning, considering a position should be permanent for an applicant only. In this way your boos may be willing to write sth as you reqiure.