Rebate Accrual Rate in Scales

Dear Gurus,

Client is offering rebate based on the quantity. like.

if invoice material qty is 100 bags accruals rate is $10

if qty is more than 100 bags, accruals rate is $ 12

if qty is 200, accruals rate is $15.

but i can not enter the scales in the accruals column, only one value can be maintain, other scale fields are grey out.

please suggest, with thanks.

regards,