Javi,
double precision will always be approximate - and obviously 8200.14 "rounds
to" 8200.13999999999999999! Try making your C function (or do it directly
in SQL) return a DECIMAL rather than DOUBLE PRECISION. I'm not certain if
it will work, but you could try something like

select nombre , orden_carga , CAST(importe AS DECIMAL(15,2)) from v_excel_oc

HTH,
Set