Under an incremental time scale the employer cannot hold increment of an employee without reason. But iif there is justification, he can do so. Still, holding increment of Dearness allowance is not legally maintainable.
Why should an employee be given increment? Two reasons, one to reward the performance of an employee and two, to compensate the increase in the cost of living. The former is evidenced by an increase in the basic salary and other allowances whereas the latter is to be given as an increase in the DA. If the performance of the employee is not upto the mark, then the employer can hold increment on basic salary. It should be communicated to the employee also that there are certain areas where he should require improvements and therefore, no increment can be given to him. But since the second one, increase in DA is a legal responsibility based on the cost of living index, if you hold it, it may fall on you. The employee may suffer a lot to sustain and that could make him even less productive.