Institution is Threatening For money

I enrolled in a 6-month (24-week) course with an agreement that if I got a placement within 6 months of course completion, I would pay 21% of my CTC as a fee. During the course, the institution extended the duration to 7 months (28 weeks) without formal agreement changes . I completed 26 weeks, exceeding the original 6-month duration. I got a job 9 months after course completion, beyond the 6-month period stated in the agreement for fee liability. However, the institution is demanding around 2 lakh INR, claiming termination due to not completing the extended course duration. Their communication has been limited to WhatsApp and phone calls; I haven’t received any formal notice or email. I had given a guarantee cheque of 2 lakh INR, and one cheque is expired its due date. They are now threatening legal action, even though the course extension was not part of the original agreement, and my placement occurred after the stipulated timeframe. I believe the demand is unjust, and I am considering legal advice to resolve this.