Legal notice

Hello sir, i have resigned from a company after 3 days of working because of some family problems.i discussed with them.They said me that i can leave.and also returned my documents .at the time of joining i had signed a service agreement of 15 month else i have to pay 1 lakh. now they are sending me a mail after 17 days of resignation. as a notice and demanding the payment of 1 lakh else they will file a case on me. but at the time of resignation they haven't said anything about this.so i already resigned from the company.i also have the resignation mail. the agreement was not on the stamp paper. that is a copy of some stamp on a plain paper. was it the legeal agreement (on white paper.). if it is legal what shoud i do.i had only attend the company dor 3 days only.inside the agreement it is mentioned that total training amount is 1 lakh.but they haven't trained me.total attended day is 3. i have to take it seriously or not? please suggest me..the agreement on the plain paper is legally valid? sir please help me.