Partner - Risk Management
Head - Human Resources
Legal Analyst, Hrm
Surprise that your present employer had promoted you in-spite of you resigning from your post. You have mentioned that they have emailed you about your promotion. Forward that email to your personal id and take a copy of the same, if needed. This might also be a retaining strategy which they would have tried.
9th August 2016 From India, Bangalore
Or as Mahesh says, it may be a retention strategy.
In either case, why will they give it to you ? You are not staying, so why will they actually promote you.
There is no,legal basis to demand the letter, so forget it and move on.
10th August 2016 From India, Mumbai