@ Ashish,
At times child like innocence may be more effective than armaments.
Line management can ease out the employee from a situation. HR can make positive/favorable recommendations. Legal cell can cite weakness in the case and can recommend that company may loose in court of law.
Employee should be smart enough to sense what shall work in his favor.
Child like innocence may work better than protest, agitation, appeal, request, legal citations etc.
Your line management and HR might be genuinely interested in easing you out of the situation and they may have the authority to make recommendations. Or your seniors may not have any say or authority in the matter at all.
You have to use your judgment. If you are confident that you can resolve the matter in your favor by providing the copy of document in question you may.
Another option is if HR or HO has not asked for the copy, you may continue to cite from your resignation that you are separating to peruse higher studies, till HR or HO demands the proof. Apply your judgment.
The lawyers at your location must be aware of precedence’s in your company and may quote some earlier judgments in favor of employees, in similar cases. Lawyers do meet their clients till late evening hours.
We sincerely wish that you should be able to resolve the matter without being dumped in courts, by applying rapport, goodwill, and exceptional levels of persuasion, persistence, negotiation, reasoning skills. This shall be the quickest and easiest way out.