Chelsea are among clubs keen on the French youngster.
"The bid wasn't sufficient," del Nido revealed.
"The idea is that he continues growing here at Seville, just like he did last season."
"There exists an obligation that if a formal, written offer of over six million Euros arrives then we must accept it and sell him, or match it and keep half of his economic rights," added del Nido.
"But they haven't done it in writing, which they must do."