Fair Data Non profit organization - Salary not paid

I've been searching for jobs past few days now and on the **th of February I received a massage on WhatsApp from "**********" regarding a Data entry work at home job , I asked If there is a registration fee that has to be paid or a penalty in case the work is not done on time , they said there is no amount to be paid , I agreed for the job after making sure I have nothing to pay for , On the same day at * they sent me the project and said I have *-* days time to submit the project . The next day that is *st March at around * , *-** p.m. I was done with the work and submitted it to them and when they had to pay me the salary for the work done they told me that , in order to receive the salary I'll have to pay Rs. *** , whereas in the beginning they didn't mention about it , now that the works done they haven't paid me the salary and are saying that you have until *nd March to pay Rs. *** to get your salary