Arsenal are set to make their second sale of the summer ahead of the close of the transfer window. The Gunners have already completed a £34million sale of Emile Smith Rowe to Fulham – a club-record fee for their London rivals. Arsenal have added David Raya and Riccardo Calafiori to their squad already and are looking to cap off their transfer window with a deal for Mikel Merino, who has already completed his medical. Arsenal are set to pay just £6.8million up-front for Merino after agreeing a favourable payment structure with Real Sociedad. Merino’s £27million fee is set to be covered by their…