HOW TO DO RATE DIFFERENCE ENTRY

FOR EXAMPLE I PURCHASE GOODS AMT 50000 (CGST 4000 + SGST 4000)
TOTAL BILL AMT 58000 . NOR SALER GIVE ME CREDIT NOTE OF RATE DIFFERENCE 1200 (CGST 100 +SGST 100) TOTAL AMT OF CREDIT NOTE 1400. HOW TO MAKE ENTRY OF THIS TRANSACTION??.AND IF I RETURN GOODS THAN HOW TO MAKE ENTRY OF THIS TRANSACTION??