I am from India, and working with an MNC. My Company sent me to Germany for an onsite assignment..?

with a work permit of 3 months and told me that work would not be of more than 3 months. Now I am here and now they are asking to stay here for 6 months. Can they force me to stay here for 6 months as earlier they didnt told me that work would be of 6 months..I have not signed any legal agreement that says my stay can be extended.. Would much appreciate if somebody can solve my problem. God bless..thanks