Hello Ankur,
In, in my view BAPI_MATERIAL_AVAILABILITY gives the picture filtered result. If you really want to know what was the amount of quantity that passed through after allocation check, you need to use the subset BAPI of the above mentioned BAPI which is BAPI_APOATP_CHECK. This is the BAPI which does the RFC call to APO system to get the data and update the schedule lines. So, you need to concentrate a bit on this one. Please look out for the internal table S_APORESX which has the split of all the checks that has been performed in APO. So, the record which has the value against the RESIND value = 6 would give you the necessary allocation quantity that you're looking for.
Hope this helps. Please let me know if you any questions, otherwise close the thread.
Babu Kilari