Why doesn't "koverto" take the accusative in this sentence? Thinking about some of the examples on Lernu: "La muso saltas sur la elefanto," is a lot different than "la muso saltas sur la elefanton," the difference being in whether the mouse is on the elephant and is jumping or if the mouse is jumping onto the elephant from somewhere else. Same sort of thing: was he inside the envelope when he hid the money, or did he hide the money in(to) the envelope? Maybe I'm just over-analyzing this sentence, but it looks like it should read as, "Li kasxis la monon en la koverton."
PIV has two examples that seem to imply that both work: en polvo mi kaŝis mian kornon (from source 'X', which I think is the Bible translation); la virino rapide kaŝis la manĝaĵon en sian bakfornon (by Zamenhof)
Hmm, that would be more like "He hid the money into the envelope". I think it's because "kaŝis" is not actually describing the motion of the money into the envelope.