As awful as this is, don't missionaries sign a vow of poverty? There have been a few threads on this over the years. Here's one from 10 years ago:
https://www.jehovahs-witness.com/topic/106721/bethel-vow-poverty-documents-scanned
One of items is:
"6. To turn over to the local organization of the Order all income received from any work or personal efforts in excess of my necessary living expenses, unless released from this vow by the Order;"
So, while I agree this is a horrible practice, it may very well be that this missionary actually agreed to that particular condition.