Dear friend,
There is no mention about the conditions of the bond.
Probably as an existing employee of the same university, you might have joined the PhD in the same university. Therefore, the university has two roles, one as your employer and the other as the institution conducting the program and conferring the degree. In that case you would have obtained prior permission to do your PhD without detriment to your duties in the university as an employee.
If so, whether the permission granted before joining the research program was unconditional to the extent of not requiring any bond?
If the permission granted by the employer/university in 2015 was devoid of bond condition, in my opinion, new such condition cannot be added later in 2019.
Since your position as both an employee and research scholar of the same university is vulnerable and more precarious, I don't think it is correct to hold that having signed the bond once, you cannot question its veracity when the very bond itself is an after thought of the university as the employer. Under this situation, it is better for you to be patient till you get the doctorate and resign afterwards. At that time you can ask for waiver or question it in a Court of Law.
You can consult your research guide, if he is reliable and a practicing civil lawyer as well.