There is no mention about the capacity of your employment - whether a workman or otherwise. That apart, It sounds very odd to me in the first place that any condition restricting the employee's exit on his own by means of a formal resignation within a certain period after his periodical pay-hike could form part of an "NDA" which is enforceable only on its registration under the Registration Act,1908.
Secondly, the act of exit by a formal resignation is covered by the conditions of notice and acceptance of resignation by the employer.Therefore, the condition that no exit within 6 months from the date of periodical salary hike would be totally unreasonable for the hike is normally based on past performance only.
Thirdly since the resignation mentioned in the post is a prospective one taking effect from a future date only after the six months period from pay hike and expiry of the notice period of two months, there is no breach of the contract of employment.
Finally and even otherwise the encashment of leave on formal exit is a matter governed by the leave regulations of the organization or the applicable Labour Law depending on the employment status of the outgoing employee as the case may be. In any case, no instantaneous modification of the rule according to the whims and fancies of the employer is permissible under Law.
Therefore, the statement of the HR person is totally incorrect and the refusal of leave encashment on the basis of breach of the so called "NDA" is totally illegal.
27th July 2017 From India, Salem