AGRICULTURE - COMMON ORGANIZATION OF THE MARKETS - MILK AND MILK PRODUCTS - BUTTER HELD IN PUBLIC STORAGE - SALE AT A REDUCED PRICE TO PROCESSING UNDERTAKINGS - RULES ON SECURITY - FAILURE TO COMPLY WITH THE PROCESSING OBLIGATION - RESPONSIBILITY OF THE SUCCESSFUL TENDERER - RELEASE OF THE SECURITY - NO EFFECT - FORCE MAJEURE - CONCEPT - DUTY OF THE MEMBER STATES TO RECOVER THE DIFFERENCE BETWEEN THE MARKET PRICE AND THE REDUCED PRICE WRONGLY GRANTED - BURDEN OF PROOF ( REGULATION NO 729/70 OF THE COUNCIL , ART . 8 ; REGULATION NO 232/75 OF THE COMMISSION , ARTS 6 ( 1 ) ( C ), 10 ( 5 ) AND 18 ( 2 ))