Half the page half the change?

I paid a friend of mine to write a paper for me for my final. Initially, he was going to write a full page on two different films and compare them. Then he said he could only write half of the page since he didn't see the second film. I was originally going to pay him $100, but now I'm thinking $50 because he only wrote half a page and I had to write the other half.
Should I pay the full amount or only half?
Half the page half the change?
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