Once the tribunal has made its decision on a case, there is normally nothing left to negotiate.
The most common exception to this rule is where the tribunal’s decision is only on liability. Once, for example, a tribunal has concluded that Bill was unfairly dismissed, it is often possible for Bill and his ex-employer to reach agreement on remedy. The tribunal will frequently assist with this, by also giving guidance on matters affecting the remedy decision. For example, a tribunal will often give a judgment in the form “Bill has been unfairly dismissed. We consider that he contributed to the dismissal to the extent of 20%. A remedies hearing will be listed, unless the parties inform us that it is unnecessary.”
It is always, of course, possible to return to the tribunal for them to decide the proper remedy, if agreement is not reached.
Negotiation is rare where the whole case has been decided by the tribunal, but it is not impossible. For example, where part of the claim is for pension loss, all the tribunal can award is a lump sum reflecting that loss. But the parties can agree that this sum, or an equivalent sum be paid into the pension fund itself. The fact that this sort of post-decision deal is unusual does not mean it is impossible if both parties want it.