So here's the story:

After 1st year living in residence, I decided to live with a guy, a classmate of his, and the roommate of the classmate. He and his classmate found the house and asked if I was still interested, I said yes.
The plan was that due to the zoning bylaws in the area, there could only be a maximum of 3 unrelated people on the 2-year lease at any time, so I never actually signed the lease, though I was still considered a "permanent" member of the house.
 2nd year I moved into the house, and I was told by the original guy that rent was $400/month+utilities since the total rent for the house was $1600/month. He also said that I was to pay him, since he'd already given our landlord postdated cheques.

I was fine with paying that, since it's pretty cheap considering what I got for it, and I have been paying that for the last few semesters that I've lived here (now on my 3rd, though I was paying the balance when someone else was subletting my room for less). 

Due to a massive fuckup on my part, I'm now a year behind the original housemates, and "off stream" from them, so I will never be in school at the same time as them again (My university alternates between 4 months at school and then 4 months on co-op). 
Last night I was talking to one of the original housemates who is on a co-op term but still trying to find a job in the area so he can live here. He accidentally let slip something about "375" and when I pressed the issue, he told me that rent was only meant to be $375/month+ utilities, as the house rent was only $1500/month.

My question now is: what should I do to get back at the person who has been ripping me off /how should I approach this issue? (They don't know that I now posses the information that they have been ripping me off)
user
– Just found out that a person whom I thought was a friend has been ripping me off for rent
assistant
So here's the story:

After 1st year living in residence, I decided to live with a guy, a classmate of his, and the roommate of the classmate. He and his classmate found the house and asked if I was still interested, I said yes.
The plan was that due to the zoning bylaws in the area, there could only be a maximum of 3 unrelated people on the 2-year lease at any time, so I never actually signed the lease, though I was still considered a "permanent" member of the house.
 2nd year I moved into the house, and I was told by the original guy that rent was $400/month+utilities since the total rent for the house was $1600/month. He also said that I was to pay him, since he'd already given our landlord postdated cheques.

I was fine with paying that, since it's pretty cheap considering what I got for it, and I have been paying that for the last few semesters that I've lived here (now on my 3rd, though I was paying the balance when someone else was subletting my room for less). 

Due to a massive fuckup on my part, I'm now a year behind the original housemates, and "off stream" from them, so I will never be in school at the same time as them again (My university alternates between 4 months at school and then 4 months on co-op). 
Last night I was talking to one of the original housemates who is on a co-op term but still trying to find a job in the area so he can live here. He accidentally let slip something about "375" and when I pressed the issue, he told me that rent was only meant to be $375/month+ utilities, as the house rent was only $1500/month.

My question now is: what should I do to get back at the person who has been ripping me off /how should I approach this issue? (They don't know that I now posses the information that they have been ripping me off)
user
1st year roommate and I were ripped off by someone who has been ripping me off for the past year
assistant