【Deduction】 , 【abatement】 , 【rebate】 , 【discount】 are comparable when they mean an amount subtracted from a gross sum.
【Deduction】 is interchangeable with any of the others but not without some loss in precision.
An 【abatement】 is a 【deduction】 from a levied tax or impost.
A 【rebate】 is an amount deducted and returned after payment either in adjustment of an overcharge or to gain a competitive advantage.
A 【discount】 is a 【deduction】 from an amount owed or a price asked in consideration of a cash or prompt payment. It also may denote an advance 【deduction】 of the amount of interest payable on a loan or note from the time the loan is made or the note purchased until the due date.