There is probably some truths and half truths from both parties.
Bamber hunting for money from Nevill so soon after the massacre is incriminating. Bamber admits hunting for the wallet.
Bamber hunting for the wallet highlights that he knew Nevill kept a lot of money in there. Which is not surprising as people often keep lots of money in wallets.
AE will be correct in that the wallet had £400 - £500 in as she handed it in.
The main question is whether Bamber knew Nevill had £400 - £500 in the wallet. After committing the massacre, he may well check Nevill's wallet. But knew he could not take it back with him on the night.