Wrong Rebate agreement determination in Billing document

Hi Expert,

I need your help and suggestion to resolve following issue

I have created on Customer 5% agreement validity period is 06.08.2013 to 14. 08.2013 .i.e. Agreement No. 20

and another Customer 10% rebate agreement validity period  15.08.2012 to 31.12.999 i.e. Agreement No. 21

In # 255751246 prepared on 14.08.2012

But system determine wrong rebate agreement no i.e. 21 which is not valid for period

Please let me know how it is possible to determination..

thnks

Sachin v