In the timing report, the slack for the fastest path analysis of the OFFSET OUT constraint comes from the following expression:
clock arrival + clock path + data path - clock uncertainty
This expression does not make sense for a "slack" because it is not a difference between "requirement" and "actual".
This just calculates a "delay" value for the fastest path in the fast corner.
When the result of this expression is a negative value, it will be marked as a violation, which could cause confusion as it is actually not a timing error.
Starting in ISE Design Suite 12.1, this "slack" has been changed to "delay" for fastest path analysis of the OFFSET OUT constraint.
In versions prior to 12.1, please ignore any negative "slack" value in the fastest path analysis.