a couple of points...
the FA have brought this uopn themselves. In 1999 they introduced the 90 min rule, good old Howard Wilkinson thought that it would help devleop local talent with their local clubs , this was meant to prevent big PL clubs aquiring all the best talent up and down the country . This meant that city and United could not sign any youngster who lived more than 90 mins drive away . But this did not apply to foriegn players..so we now have a situation where Chelsea , United , arsnela have feeder clubs abroad who spot and develop talent within their locality , at a much lower costs. Then when suitable players are recognised they can be brought to this country for a lower fee..rafeal and Fabio were bought for 1m each from the brazilin feeder club , Song , denilson bought from the german feeder club ...it wouldnt be possible now for United to sign a kid from London , or city to sign a youth player from Scotland...
the legalities with regards contracts are not standard across different countries, and are subject to many different regional and local FA laws
as for the two cases brought by the french clubs...I understand the facts to be ..the city player was under a pre contract ..although how legaly binding that is is hotly debated
the lad Pogba , never signed a contract