Agreements are there to be amended by Rentoza as they see fit

Had a few subs over the last 2 years with Rentoza ,and it's been okay,now that I have paid my last sub for my Tv they all of a sudden have fallen on their heads to say that after 12 months of paying I have a buy out balance which by the way is No where stated on my agreement. Now i being told they can ament the agreement as they please and I must fulfill that 13 thousand payment. When the agreement was singed by myself and the a manager was there never a mention of a buy out amount. Wow I am blown away by the slyness of this company.