Bank generally ask for the source of funds/income when you open an account not on each specific transaction.
So they would probably just have "donations" recorded for EFF and Afriforum on their profile. They might ask again if a transaction seems "irregular" according to them, but that is very subjective. And everyone involved likely do large transactions regularly so it won't be flagged as something irregular.
How it would work practically, If someone even noticed was they would call Afriforum and ask. "Hi you normally get donations of R150 from your members, what is this R300k amount.
Then Afriforum would just reply. "O legal fees."
Done. That would be it. No deep investigation are done or anything, that is very expensive and takes specialized man power.
Somehow I messed this up. I just meant to edit the above post to make it more complete.
Source of funds is not specifically who the funds are from, but generally what kind of business you are in. It is for money laundering purposes to identify "irregular" transactions and build a profile. The specific transaction details is there for the specifics, if that is needed.
For a normal person your Source of funds would be salary/wages.