Despite the advice given by Polonius we sometimes need to "borrow" during the subtraction process, so while 345-122 is achieved without "borrowing", 322-145 needs two acts of "borrowing".

Having stated the above, please find the largest set of 3-digit numbers, such that for any pair(a,b; a>b) of members of the set the operation a-b cannot be done without borrowing.

Based upon a NSH contest problem.