Paid temporary job as research assistant I'm a master's degree student. A professor of mine offered me work on some parts of his paper that I have good knowledge about, because of my background. He said he'll pay me in return, and acknowledge me in his paper. I accepted and suggested a 10 EU/h work and he accepted. I suggested that rate mainly because this is my first experience of this kind (and hence, I don't know the average payment rate) and I thought it would at least take 40 hours and the final money will be enough for me to minimally live where I live. Now it turns out that (I guess, unfortunately?) I was faster than I thought and the work was done in around 12 hours. At the same time, of course, I'm not going to lie about my work hours just for the money. What do you think I should do? I had counted on that money, and it seems that my speed is turning against me! 

PS 1: He's friendly with me; I'm just thinking bringing this up might make him feel I'm overstepping my friendship with him, and that's the last thing I need him to think.

PS 2: The paper that he's working on is part of a big university project that he's here for. Accordingly, the payment will be done from the budget of the project, not his pocket (or maybe they're the same?)

PS 3: The contract will be prepared by the university in a few days. If I ask him to raise the money and he accepts, he'll probably be able to convince the university for that.
He will almost certainly agree to use your talents for the remaining time, and pay you the full sum. Take some additional time to check your work; speed and quality will matter. Then talk to him about your predicament, and request to be kept on for the agreed upon hours doing whatever it is you agree upon. He should be impressed that you do good work quickly. Do more, you may acquire an ongoing arrangement.

Note that this outcome is not guaranteed: humans can be strange creatures sometimes. That said, it is virtually guaranteed. If you have a good relationship, I suspect you can proceed without fear.