Negative Float is created by software that under "no latter than" constraints, when these constraints are not achievable still insists on modeling the non-achievable and assign those dates to the Late Dates, fooling CPM calculations into modeling the impossible "late" being earlier than "early", "tomorrow before yesterday".

The best way to handle contractual criticality is to use software that displays criticality without enforcing the non-achievable late dates, software like Spider Project. BTW I would call the concept "stupidly simple", so obvious most people miss it and follows wrong logic; if the constraint is unachievable then the critical activities Late Dates must equal the Late Dates that can be achieved which under these circumstances equals the Early Dates. Some software developers when programming FNL constraints forgot to include in the algorithm the implied constraint, that the impossible shall not be modeled as a float value, just wrong mathematics applied to the concept of float.

Out of sequence algorithms can reduce available float and under wrong Negative Float functionality it will impact negative float values. Out of sequence links can be SS/SF/FS/FF links, it is not limited to FS links.

Negative float is created in one of two ways:

1) You have a "no later than" or "mandatory" type constraint on one or more activities which the schedule cannot achieve

2) You have OOS (out of sequence) activities -e.g. an activity has an actual start but with a FS predeccesor which as not yet finished.

In the case of 1) You can fix the negative float by removing the constraint(s). Some clients insist on using these constraints e.g. for contract milestones. If that is the case, set them and show your negative float for reporting purposes, but take them off when using the schedule to actually manage the project.

In the case of 2) Different schedulnig options (retained logic, progress overide or actual dates) from the schedule options dialogue box will give you different results (both for float and for early dates), but the only way to properly solve the issue is to ammend your logic to reflect reality. -If activity B has actually started before Activity A has finished, then clearly the FS link you had between them has proved to be innacurate and should be deleted / ammended.