The original contract between the club and sala was rejected by the premier league as sala had insisted on the signing on fee in one payment to which the club had agreed.
EPL rules allow that a signing on fee must be paid across the length of a contract so in salas case a 3 year contract meant 3 seperate annual payments.
Sala had not signed the new correct acceptable contract to the EPL prior to his passing although Nantes could argue that a football club with a decent football secretary would have known the rules and not produced the first contract.
the picture of sala signing his contract was a publicity stunt to placate the fans at the time.