Figuring out rent in a townhouse?

I'm renting a townhouse with three of my dorm roommates. Basically, they get the large 12'by 18' rooms with walk in closets, and I will get the small 9' by 9' room with the single 2'wide cupboard for clothes. One girl is incredibly cheap, and argued that she wanted the small room for $150 less, which the other girls and I agreed too. Well, circumstances changed, and I ended up with the small room. It actually works: My college program ends in April and we move in August, and theirs will continue during the school year. I will be working, and they will be in college with more stuff (they're in graphic design, so they have monitors and electronics galore. Basically they need more space). Well, it was understood that whoever gets the small room will be paying $150 less on rent. Now, the girl who was originally going to get the room is saying that because I'll be working I should only pay $30 less, versus them who will be in college and scrimping. The girls and I tried to explain that we already agreed on $50 more per larger room, but the cheapskate won't budge. However, she said that if I make a big deal of it I'll be paying more, because it's only fair since I have a job. How do I explain to her that she can't turn things around to suit her? We don't have anything down in writing, but the other girls are vouching for the $150 reduction because that's what we agreed on? This girl is also threatening to kick me out and STILL charge me rent (25% of the entire rent) until I find a new roommate for them. How can we tell her that it's not fair and that she can't just change things around like that??