The Missing Digit Puzzle

Here is a simple puzzle you can try on friends. Depending on your skill at blather and your friends's curiosity, this can produce a severe sense of puzzlement.

1. Choose a 3 or 4 digit number at random (I chose 3435).
2. Make a new number by scrambling the digits (I got 5433).
3. Subtract the smaller number from the larger (1998 for me).
4. Cover up one of the digits, (but please, don't cover up a zero!) scramble the remaining digits, and report them. (I hid the 1, and reported 899).
5. The missing digit was...(1, for me).

Try this procedure yourself, and see if you can figure out how the mathemagician can determine the missing digit!

I give up, let me see the solution.