MSP negative float and constriant

Vijay,

(Presuming "MSP" in your title means Microsoft Project)

1. If the whole program moved to the right after removing a FNLT constraint, then the project is scheduled backwards, and you are in a heap of trouble.  Most people who use terms like "float" and "finish on or before" don't schedule like that.  (Check the Project Information dialog for  "Schedule from: = Project Finish Date" and note the Finish date.)  

2. If only the final milestone moved to the right after removing its FNLT constraint, then:

• You have left the project schedule option for "Tasks will always honor their constraint dates" checked (this is the default setting.)  I would normally un-check it to prevent late constraints from overriding logic.  With the box un-checked, removing the FNLT constraint wouldn't "move" anything;  only the late dates and the float, not the scheduled (early) dates, would be affected.
• Ignoring other constraints and funky logic, check the finish milestone calendar and the project's "hours per day" setting.  These have a great impact on the interpretations of durations and float.  For example, a milestone with a 24-hour calendar in a project with default settings (8hr/day) only needs to accelerate by one calendar day to recover 3 "days" of negative float. (This is not an issue in P6 because P6 calendars each include their own hours/day setting, while MSP only has one for the whole project.)
• (Obviously) make sure there are no manually-scheduled tasks.

 Good luck, tom