Thousand decimal point problem

Question

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