AGRICULTURE - MONETARY COMPENSATORY AMOUNTS - BASIS OF CALCULATION - VALUE OF THE PRODUCTS CONCERNED - BUTTER FROM STORAGE - SALE AT REDUCED PRICE TO PROCESSING UNDERTAKINGS - REDUCTION OF MONETARY COMPENSATORY AMOUNTS - CONDITIONS - REDUCTION IN VALUE OF GOODS OWING TO THEIR COMPULSORY DESTINATION - RE-ASSESSMENT OF COMPENSATORY AMOUNTS - LEGAL BASES ( REGULATION NO 974/71 OF THE COUNCIL , ART . 2 ; REGULATION NO 1259/72 , ART . 6 ( 1 ) ( C ) AND ( 2 ) AND ART . 20 )