Hi! Thanks for the question!
The variable pay is definitely a part of your CTC and should be payed to you, however, it is dependent upon the performance of the employee and the company has the final say in such matters.
As you have mentioned that the employer has sent email to you and many others agreeing to pay the same, the case is in your favour and you can issue notice to them demanding your rightful variable bonus/pay. If the company does not pay, you can file a civil suit in the court with competent jurisdiction.
You will have to tug on the issue of you resigning and that will need a good lawyer in the civil courts.