Rafael said:
The Areas has a 1:N to Documents

You can use this link instead of doing a select() again. What is the reference field name to Documents?

If the name is Documents then you can do something like

sum(Documents[Document = "something"].Budget)

Question: Why don’t you use the Area_Documents link to get the data you need? I don’t know all of the fields but you can also maybe do:

Area_Documents[fieldname = "something"].'Plan Value'

Take a look at your code again:

sum(Area_Documents[Document = "General"].'Plan Value')

Looks like your are using the wrong field to filter. Shouldn't it be:

sum(Area_Documents[Areas.Area = "General"].'Plan Value')

Good thing I double checked. I didn't know Areas is a field of a linked table.