The key fields of RESB are
RSNUM
RSPOS
RSART
and these fields are available with both your tables.
so if you want to delete it_resb1 where the same
RSNUM
RSPOS
RSART in <FS> has VBKZ = 'D'
you write the following code
field symbols : <lv_vbkz> type any,
<lv_rsnum> type any,
<lv_rspos> type any,
<lv_rsart> type any.
loop at <fs> assign <fs_wa>
assign component 'VBKZ' of structure <fs_wa> to <lv_vbkz>.
if <lv_vbkz> is assigned.
if <lv_vbkz> = 'D'.
assign component 'RSNUM' of structure <fs_wa> to <lv_RSNUM>.
assign component 'RSPOS' of structure <fs_wa> to <lv_RSPOS>.
assign component 'RSART' of structure <fs_wa> to <lv_RSART>.
delete it_resb1 where rsnum = <lv_rsnum> and rspos = <lv_rspos> and rsart = <lv_rsart>.
endif.
endif.
endloop.
this should solve your problem.