Roommate is asking me to pay next six months of when I am leaving.

Location: Florida, USA. For context I am living at the apartment since last 4 months including current month. I am moving this month and and I had never left my share of the rent and paid it on time. But since I am not on the lease and landlord has no idea that I am living here, my roommates who are on the lease asking me if I don't find anyone to replace me I have to pay next six months portion and to make it legal they are asking me make and sign notary. We never discussed this part that I had to pay six months of rent. And I don't know what to do. Before coming here I already told the guy who gave me this place that I will be leaving in this month. I don't know what to do in this situation. Should I negotiate. Can I leave without paying this huge amount. What can I do. Please help