Stalin was desperate for a pact with Hitler in order to ensure the Russian army had sufficient time to modernize. If von Ribbentrop was shot down he would do everything short of flying to Berlin and prostrating himself before Hitler to ensure he got the pact.
Stalin would profusely apologize, offer to pay an indemnity to von Ribbentrop's family, have the soldiers responsible for the shoot-down publicly tried and executed (probably with the German ambassador to Moscow as an observer to the proceedings), and then ty to reopen negotiations.
From Hitler's perspective the death of von Ribbentrop would only further demonstrate how little the Bolsheviks could be trusted. He would still press for a pact, but would insist on more concessions from the Russians.