Company now paying incentives as promised how can we proceed

Sir i have been recruited in January month for doing a job as intern in a private company for doing currency trading and we have given an offer letter where it says that we'll be paid upto 800 $ as incentive but after working in that company till April month we have not been paid and when we went to enquire about it the hr said that we'll be paid according to new plan which decreases our hard earned incentives as per the previous told variable pay and now they had given us new offer letter in which it says that all the previously told variable will be cancelled with our sign on it please help