Company asking for repayment of 15000 dollars and do not have repayment agreements to refer:

I left a company in a period of 1 year 5 months, due to too much toxic culture and suppression by my manager and his boss and due to long time required to find a job. After leaving I am reached by the company to pay back $15000, 33% of the original sign on bonus, because according to them it is earned in 6 months increments and I would have earned the full bonus, had I stayed with the company for full 1.5 years. And if not payed in 30 days they will send it to collection. Now for me to first refer any such signed agreement , I asked them to share any signed agreement, but they shared only the agreement template(unsigned) and after several followups they said, they do not have any signed agreements in system