Here are my two cents on this.
The ESOP value has been declared "NIL" by the trust/board of directors. This would have been declared during the acquisition based on the liability show cased in the books. This would be one reason.
Secondly, having said that, when it is declared NIL, the associates/employees shouldn't have signed/written off the ESOP options. By doing this, they are left with no choice to proceed legally as the document of write off being signed by the respective employee.