DATA: storno(10) type c.

if BKPF-AWTYP = 'VBRK'. "Фактура: данные заголовка
  select single bkpf~BELNR
  from bkpf inner join vbrk on bkpf~AWKEY = vbrk~vbeln
  into storno
  where vbrk~SFAKN = t_bsid-awkey.

elseif BKPF-AWTYP EQ 'RMRP'. "Входящий счет: заголовок документа
  select *
  from rbkp
  where rbkp~STBLG = t_bsid-awkey(10)
    and rbkp~stjah = t_bsid-awkey+10(4)
    and rbkp~BELNR < rbkp~STBLG.

    DATA: rbkpAwkey(14) type c.
    concatenate rbkp-BELNR rbkp-gjahr into rbkpAwkey.
    select single bkpf~BELNR
    from bkpf
    into storno
    where bkpf~AWKEY = rbkpAwkey.
  endselect.

else.
  select single bkpf~BELNR
  from bkpf
  into storno
  where bkpf~STBLG
endif.