Help with Stored procedure

Hello experts

I am trying to create SP which achieves the following:

Prevent users from changing 'Main Sales Employee' (OOPR.SlpCode) field unless field is currently "-No sales Employee-"

But SP must not apply to user '6' (User 6 can always change this field)

This is what I have so far but is not functioning correctly:

IF @transaction_type = 'U' AND @object_type = '97'

and exists(select t0.SlpCode from OOPR T0 where T0.SlpCode != "-Main Sales Employee-"

and T0.UserSign != '6')

BEGIN

SET @error = 1

SET @error_message = N'Authorisation required'

END

any help would be appreciated

karen