Rathish,
Use exit USEREXIT_AVAILABILITY_OUT (RV03VFZZ) or EXIT_SAPLATPC_002 (ATP00001). Please refer SAP note 1644443.
You need to be very cautious using this as you need to round down the confirmed schedule line as per value maintained in material master. Example we have maintained 1280 in material master but stock available is 49000. But confirmed is 48640 which is multiple of 1280 but less then 49000. This is done to avoid over-confirmation. Please do good amount of testing considering all scenario's in your business before moving this to production.
I hope this resolves your issue. Let us know in any case.
Regards
Rahul