Is this discount method rite or wrong?

HELLO EVERY 1,

I HAD A QUESTION TAT AS M A RETAILER AND I PURCHASE  FINISHED PRODUCTS FROM MASTER FRANCHISEE AT A WHOLESALE DISCOUNT AS HE SAY IS 20 % BUT WHILE BILLING; HE THE METHOD HE USE IS BACKCALCULATION FOR 20% HE PROMISED TAT IS ( MRP DIVIDED 120 MULTIPLY BY 100 = OUR DEALER PRICE )  SUPPOSE A PRODUCTS MRP IS 10,000/- DIVIDED BY 120 X100 = 8333.33 IT COMES AND IF WE STRAIGHTLY LESS 10,000/-  (-) 20% = 8000/- .THERE IS A DIFFERENCE OF AROUND 333.33 IN BACK CALCULATION . WHICH PROCESS IS RITE DIRECT LESS FROM MRP OR BACK CALCULATION METHOD ? I WANNA KNOW FROM EXPERTS PLZ REPLY