Dear friend,
There is no mention of the conditions of the bond. Probably, as an existing employee of the same university, you might have joined the PhD program at the same institution. 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 pursue your PhD without detriment to your duties as an employee. If so, was the permission granted before joining the research program unconditional to the extent of not requiring any bond? If the permission granted by the employer/university in 2015 was devoid of a bond condition, in my opinion, no such condition can be added later in 2019.
Since your position as both an employee and research scholar of the same university is vulnerable and 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 afterthought of the university as the employer. Under this situation, it is better for you to be patient until you obtain the doctorate and resign afterward. At that time, you can ask for a 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.