Asked for resignation

I was working in Finance bank Since from 3 years ,I was permanent employ there,got my campus selection in 2016 ,started as management trainee,and promoted as Asst.manager . I am handicap by one leg,I was good at my performance in sales team. I changed my department in credit after i completed my management trainee period ..i totally treated differently i never got any work similar to my colleges ,they gave me work which was done by my juniors..as they said for your safety we don't late you go to field 1 year they gave me desk job which was junior level job ,where as i was capable to work on field and did good work even similar as my colleague did ,even more than him,they ask me not go on field an we care for you suddenly on 20 dec 2018 they call me and ask me for resignation and said you performance was not good.where as it was not my fault the did not allow me to work, I asked can i complete my notice period ,they said no ,i resigned after that they cut my gratuity & salary for not serving notice period