The police may never be able to find out exactly what happened on that night. There is only one living witness and if his testimony is suspect then it will be difficult.
The police may never find that money, it could be difficult trace a smallish amount of cash like that and there are many ways he could have spent it.
The police should be able to establish if the wife knew the dead man better that her statement suggests. Unless they were exceptionally careful, there will be witnesses who saw them together and phone records.
Once there is more information about that, the story will fall into place.