My code is ok, but if have values from 0 to 999 its ok
 
Exmpl two values 500.00 and 80.00 result is 580.00
That is ok
 
Exmp if have two values   1,000.00 and 100.00 result is 100.00
 
Some help?
- select kupac as 'Kupac', 
 
- SUM(TRY_CONVERT(Decimal(10, 2), ISNULL(NULLIF(iznos_sa_pdv, ''), '0'))) AS 'UKUPNO'   
- from(select kupac, iznos_sa_pdv from mp_faktura_lista union all select kupac, iznos_sa_pdv from mp_racun_lista) as alltables  
- group by kupac