Harjan,
0FI_AA_11 the FM based extractor is BWFIAA_EXTRACT_DELTA and for 0FI_AA_20 the extractor is
BWFIAA_SREP_GET_DATA and also 0FI_AA_20 is part of EHP4(Reporting Financials 2) .Get help with the abaper to see how the FI_AA_20 is getting the BELNR field. My guess is that would be from BSEG.
Try it and let us know.
Hope this helps...