At the other Friendship Bridges #1 Nong Khai, #2 Mukdahan, #4 Chiang Rai (and should be at #3 Nakhom Phanom), have given USD $100 and received change back in USD and paid $40 Dollars, not 2000 Baht.
But, very sad that you had a bad experience at Friendship Bridge #3 Khammouane for your Lao VoA!
Did you try paying with USD $40 in crisp, clean (new) US Dollar banknotes?? before paying 2000 Baht for the Lao Visa on Arrival (Lao VoA) at Friendship Bridge 3 (Nakhon Phanom)