Сделай обратное преобразование ...

select c.okpo,
to_date(to_char(i.document_date,'MM/YYYY'),'MM/YYYY') data,
substr(c.s_name,1,30),sum(i.total),count(i.id)
from BUX_MAIN.NS_IN_INVOICES i,fin_adm.fi_sp_client c,fin_adm.fi_client_bank b
where i.id_supplier=b.id and b.client_id=c.id and i.id_state=1
group by c.okpo,to_char(i.document_date,'MM/YYYY'),substr(c.s_name,1,30)

и всe будет хорошо ...

или вот так даже лучше ...

select c.okpo,
trunc(i.document_date,'MM') data,
substr(c.s_name,1,30),sum(i.total),count(i.id)
from BUX_MAIN.NS_IN_INVOICES i,fin_adm.fi_sp_client c,fin_adm.fi_client_bank b
where i.id_supplier=b.id and b.client_id=c.id and i.id_state=1
group by c.okpo,trunc(i.document_date,'MM'),substr(c.s_name,1,30)