basic but annoying: help needed in table visualisation

Hi there
I have the following setup: 

db-table "budget"

contains planned values, plus a unique budget ID.

db-.table "purchase" contains purchase values and dates , plus a direct reference to the budget ID (as purchase is done from budget)

same with 

db-table "invoice" 

I want my final 

powerBI visual table to list all budget lines. then add values from both linked tables if there are any. if not, leave the columsn blank, not the line. 
usually, purchase happens before sales -invoice. 
so when I add purchase info to the visual, all values are displayed. 
when I add sales-invoice info as well, several purchase data is omitted, because there´s no corresponding sales-invoice data in the database yet. I still need to show the purchase-data though. 

how can I accomplish this ? 
thanks in advance