I've worked at a lab for over a year and last semester I told them that I couldn't commit as much time but that I would be in periodically and help with any lit or writing they needed. They all agreed. My post-doc just emailed me saying they want me back this semester and I would need to put in 12 hours a week. I said fine, but as my class schedule was already established, I would work around it. He said he needed "one, preferably two" 6 hour blocks a week and that "it would be good for my rec. if i say yes". So I responded that I could definitely work 6 hours on Friday when I have no classes and if, a certain week, he needed two, I could skip classes to make it work. He sent me an email just now saying "I'm afraid I need two 6 hour blocks of commitment." I'm at a loss and I don't know what to do. It looks bad not to have a rec from (1) the only research I've done and (2) somewhere i've worked for a year. I feel like he's using the rec. to make me his slave, which was fine, but this time when I really, truly cannot comply, I feel like I'm screwed.