Franchise FCStranded Sell on clauses are generally valid until the player is sold on.
I believe most sell-on clauses are valid for the length of the players contract. Since I can't believe we gave NS a 7 year contract, then the original one must've expired. On that basis I'm not sure why Orient would get anything at all.
Because the sell-on clause is in a contract between RFC and Orient.
NS signing a new contract between him and RFC can't invalidate the existing contract between the clubs.