If source is browser you can also set path with browser keywords. RPA.FileSystem does not have any keywords for typing in dialogs it can move files which are already present. If you need to use dialog box you can interact with it using RPA.Windows library.
*** Settings ***
Documentation Set path and save or open file using
... Windows dialog
Library RPA.Windows
*** Tasks ***
Minimal task
Set Filepath In Save As Dialog
*** Keywords ***
Control Save As Windows Dialog
Control Window name:"Save As"
Set File Name For Save As Window
[Arguments] ${filename}=examplee.txt
Set Value id:1001 ${filename} enter=True
Set Filepath In Save As Dialog
Control Save As Windows Dialog
Set File Name For Save As Window myfilepath.txt
It was because I had lot of windows open and this seemed to work reliably. Its not working for you? I think there can be case that there are more applications with that id, then you must set anchor or find some other reliable way.