感谢网友提供了如下信息：
Rebates are not considered taxable income. A recent IRS ruling which states that a rebate we provide you will not be considered income, but rather a lowering of the purchase price:

The IRS determined in Ltr. Rul. 200721013 that certain commissions received by real estate brokers that were passed along to their home-buying clients were neither reportable payments to the brokers nor gross income to the clients.

Source: RIA/WGL Journal of Taxation (preview) on Checkpoint

I report the 1099 income on 1040 line21, at the input sheet first list the 1099 amount, then next line put a negative number of the above amount, describe as "agent rebate, nontaxable". You may also attach a statement saying the 1099 you received is agent rebate, which is non-taxable income and should be deducted to the basis of your house.

我在网上作了搜索，查到一些关于这个主题的文章 (Following is the reference):

http://www.rehava.com/files/0721013.pdf

http://ataxprof.com/files/08-17-07_Real_Estate_Broker_Rebate.doc

http://server.housingprograms.org/docs/6/USTreasuryDepartment-IRSRuling.pdf