Hi Ajay:
In my system , 200 is being posted to GRIR (332011) at the time of MIRO and and credit of 20 is given to the expense account in question with the same commitment item which is i think as per standard since you have been 20 less by vendor so your expense has been reduced by 20 making overall budget consumption to 180 as can be seen in the attached image which shows complete trail of your scenario from PR,PO,GR,IR. Please show us the same from your system so that it could be analyzed further .Please also tell us the update profile you are using .
Regards