Weirdly the CIA actually does require case officers to get signed receipts from their assets for payments. Whether they verify the signatures is another question...
Ha! That's interesting. I think it would be funny to read some of those. "I, Mr. Ivanov, the Russian spy, got: 2 bars of gold, 5 buckets of caviar, 10 cases of vodka and 3 kilos of cocaine". Then they call the CIA headquarters complaining the cocaine is stale and they would rather have another 5 buckets of caviar instead.