Just a thought but how do we know he gave the envelope back empty? He may have put what was requested inside after showing wife, yet was still shot.
We know very little in truth. Everything comes from one source, AWs wife. We have to assume she has given a honest, accurate and truthful account as there is no other corroboration.
The official line is that on the conclusion of the first conversation AW returned inside with an envelope. The colour and size has varied in reports over the years. The envelope had PAUL written on the back of it. I don't think the police have ever stated specifically one way or the other if the envelope had anything else written on it. AW then had a conversation with VW the details of which have never been released. AW then decides (bizarrely imo) that he is going to go back outside and see if the person was still there. For some unknown reason he was and a second conversation took place which ended up with AW being shot and the envelope presumably being taken by the shooter. I've always thought AW was always going back outside and that the shooter knew this and was waiting for him on that basis.
Many years later the police have appealed for information on the envelope. VW has spoken about it but questions and responses have been tightly controlled by the police. Its not immediately clear what the police were expecting to achieve from the appeal given probably the only one who could help would be the shooter himself.
So in terms of the envelope itself there are many possibilities. AW may have been given it with something inside. There was one report where the police apparently said AW didn't open the envelope because he already knew what was inside. That doesn't fit with the general narrative though.
So possibly:
AW was given an empty envelope which he knows what it was for.
He was given an empty empty envelope and he did know what it was for.
He removed the contents before showing it to VW.
He was only pretending to be confused when discussing it with VW.
He put something in it before going back outside.
He took an empty envelope back outside as he didn't know what it was for.
He took an empty envelope back outside as he was refusing to comply with the callers demands.
He disposed of it in some way.
There is an elephant in the room for all of this though. The official line is that AW went back outside as a spur of the moment decision. If that is true then the caller had no expectation of getting it back in tbe first place so it wasn't particularly important anyway. Officially the caller had no idea AW would return outside and also no idea if he did that he would have the envelope with him. The whole thing has never really added up for me.