Seal The Deal Meaning

(idiomatic) to come to an agreement, to finalise the deal.

Example: 2014, Jamie Jackson, "Ángel di María says Manchester United were the ‘only club’ after Real", The Guardian, 26 August 2014:
  After completing a medical and the requisite paperwork on Tuesday to seal the deal, Di María said: “I am absolutely delighted to be joining Manchester United. I have thoroughly enjoyed my time in Spain and there were a lot of clubs interested in me, but United is the only club that I would have left Real Madrid for.