Low ranking university with fund VS Unsure opportunity at a higher ranking one Two years ago, I completed my bachelor degree with highest honor ranking 1st of my class in the Electrical and Computer Engineering department in my university in Egypt. Since then I have been working as a teaching assistant in the same department. I was seeking to pursue my Msc degree at a high ranking university and hence I applied for a Msc in 12 universities. I was able to gain admission(without funding) in four of them(among them was Carnegie Mellon and University of Michigan ann arbor). Meanwhile, a friend of mine helped me to get a research as position in a low ranking university in the US (No. 160 according to Us news ranking).
 In the low ranking university, I am supposed to work intensively (the lab director requires the students to work on Sundays!) on a field that is not my primary field of interest. Also, he , this adviser, expects me to continue my Phd with him and my friends in this lab informed me that he will not give a recommendation letter if I want to leave his lab later. However, I am shooting for a higher ranking university for the Phd.   

Do you advice me to go to the low ranking university with this adviser and then shoot for a higher one for the Phd? Or to wait and apply again for other universities for my Msc?

Also, is it OK to change my field of interest after finishing the Msc? will this limit my opportunists for a Phd in my primary field of interest?

Finally, how important is the Msc adviser's recommendation letter while applying for a Phd?
The answer is clear, and has nothing to do with ranking: if you want to go onto a PhD, your application will mostly depend on the letter of your research advisor. 

That being said, there are terrible warning signs, and I would not take that position. Requiring you to work on sundays (and saturdays I presume) does not make a healthy student. Even students need down times. Sure, when I was writing my thesis I worked for a month straight; but thats crunch time. During normal students life it should be fairly balanced (40-60 hours a week working, with some play). 

If you are willing to pay (another topic) go to the higher ranking schools, or wait a year and reapply to see if you can get funding at a better school. The advisor at the lower ranking school seems unethical, and I would not work for him even if he was at a school like MIT or Caltech. The fact that he wouldn't write you a letter of recommendation if you decided to leave is truly an indication that he does not have your best interests in mind: writing letters his his job. 

As so many put it here, run don't walk.