Except command doesn't work in SQL
From you comment:  


  "importe_trx" and "precio_venta" stores the same data (float) that are the prices


That means you are using float data type to store these numerical values.
Problem with the float data type is that it always store rounded values, it can not precisely represent decimal values. Each values is saved as (0 or 1)*1/2+(0 or 1)*1/4+(0 or 1)*1/8+..., so 18.42 + 24.56 can be 42.98000000001 in float terms. SSMS will round these numbers, so you will not see the difference.

To fix this problem you should use decimal data type. Never use floats in any accounting data.