i may be wrong but, i think an 8speed cassette with a 9 speed chain should not be a problem.
if it was a 9 speed cassette with an 8 speed chain it may have issues.
8 speed chains are wider than 9 speed chains. i think i've read this on this forum somewhere regarding chain compatibility on rear cassettes.
Sheldon describes it at length and in digestible detail here:
EDIT: just re-read your post and think you are talking about a 9spd rear mech with 8speed chain/cassette and shifter. if thats the case, ignore my post!
having realised that, i dont think that would be a problem either....
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