So, two things that I could see are happening here:
1) Mix wants to be a free agent and get out of that MLS contract - he’s under contract with MLS until 2018 so he could only be loaned out (like he is now). CFG maybe said to him that he’d have an easier time finding a European team if he signs with CFG who has many contacts at European clubs.
2) CFG is helping out MLS - although Mix has nothing to do with NYCFC’s cap, MLS is still paying his salary (or at least anything above what the loan club is paying). Getting him off MLS books saves MLS some cash. NYCFC doesn’t really care.