Calleri’s situation is still pretty much uncertain at this moment. Today journalist Pablo Gravellone gave some insides on Jonathan Calleri’s situation. The Boca Juniors striker is being pursued by Atletico Miniero and two unnamed Brazilian clubs. There is also English interest.
Source: FCInternews.it

how is that English clubs could register him and we cant?
If i’m not mistaken, every league has different regulation regarding foreign players, for EPL I think it’s 25 players with 8 players from England is a must and (home grown) so they has more foreign non-eu quota than serie a which can only sign 2 perseason.
that’s not the issue here. we can’t register him due to some law or rule that no 3rd party can owe a part of players contract.