Hi Sebastian,
Thanks for posting back and sharing the detailed information with us. Glad to know it works when you use a new Excel file and a new document library.
I'm afraid it requires to link to content for the custom property then it can work. Thanks for your understanding.
For the issue that when you use the name "Neto value", it didn't work, I went to test again and it still works from my side.
After checking the screenshots you shared, I found a bit difference between my test and yours.
The property name in your Excel file is Neto_value, but the number type of column in your SharePoint library is Neto value, it doesn’t have a dash between the 2 words.
However, in my test, I make sure both the property name in Excel and column name in SharePoint library is Neto_value, it has a dash.
When testing in Excel, I found that if I set name box for a cell to Neto value, it didn't work, so I chose to keep the dash.
I also did further test on my side and found that it requires exactly same name in both Excel property name and SharePoint column name. As you can see below, I created 2 columns, one is Neto_value, the other is Neto value. Only the Neto_value column shows property's value.
Given this, I recommend you open Properties list in your Excel workbook and make sure the property name is Neto_value. (I notice you miss the slash). And then try to create another Number type of column in your SharePoint document library and set its name to Neto_value, check if it works.
Thanks for your time and effort.
Best regards,
Tina