There are two very diff' scenarios -
a) The issue around the status of the 'contract' such as City's argument with UEFA. Very likely to go through the courts as a test case needs to be conducted and rules etc clarified in law.
b) They are very guilty of breaching rules through being very,very naughty in other ways - direct and obvious tapping up, illegal payments to third parties to achieve the desired player capture etc.
If its B then I agree with the other posts, if A then we will see the courts in action.
Only UEFA and Chelsea know at the mo'