Actual Amount Is Greater Than AP/Invoice

hi experts,

i have one issue, When i am doing Purchase Order, Actual amount of Purchase Order is greater than AP/Invoice,

Example,

Purchase Order For Item A is,

Required quantity 300 kg so my AP/Invoice is according to 300 kg Qty but Vendor have that item with quantity 320 kg & his Sale Order is according to 320 kg & his Invoice will be done according to 320 kg.Vendor giving me discount on that 20 kg. i have to make my AP/Invoice same as Vendor Sale Order Invoice with same Amount.so how can put that extra 20 kg amount on discount against that particular order.