Barcelona and Liverpool reportedly have an agreement prohibiting the Catalans from signing Reds players until 2021 as part of the Philippe Coutinho deal.
According to Cadena Ser ( h/t MailOnline's Tom Farmery), Liverpool insisted on such a clause. However, the clause can be broken if the Reds actively try to sell a player themselves. Mohamed Salah is said to be a target who can't make the switch to the Camp Nou unless the British side want to move him on.
This article will be updated to provide more information on this story as it becomes available.