String and Variable concatenation not working in Applescript
If you run

set user to "johndoe"
set source to POSIX file "/Users/" &amp; user &amp; "/Documents"


the result is

{file "Macintosh HD:Users:", "johndoe", "/Documents"}


Just put parentheses around the path:

set user to "johndoe"
set source to POSIX file ("/Users/" &amp; user &amp; "/Documents")