Consider the following. When clicking in "Browse" and then choosing a file, the path will remain in the original one and won't change to the one of the file I chose:

nb = EvaluationNotebook[];
DynamicModule[
  {path1 = FileNameJoin[{$HomeDirectory, "Desktop", "file.nb"}],
   path2 = FileNameJoin[{$HomeDirectory, "Desktop", "file.xls"}],
   date = "",
   res = ""},
   CreateDialog[
   Column[
    {"Path of" Style["Functions", Italic],
     Row[{InputField[Dynamic[path1], String, Enabled -> False], 
       FileNameSetter[Dynamic[path1], 
        "Open", {"Mathematica Notebook" -> {"*.nb"}}]}],
     "Path of" Style["DataInput", Italic],
     Row[{InputField[Dynamic[path2], String, Enabled -> False], 
       FileNameSetter[Dynamic[path2], 
        "Open", {"Excel files" -> {"*.xls", "*.xlsx"}}]}],
     "Date",
     InputField[Dynamic[date], String, ImageSize -> 200, 
      ContinuousAction -> True, Alignment -> Right, 
      FieldHint -> "DD/MM/YYYY"],
     Grid@{{
        Button["Proceed", res =
          MyFunction[
           path1,
           path2,
           date
           ], ImageSize -> Automatic, 
         Enabled -> 
          Dynamic[StringQ@path1 \[And] StringQ@path2 \[And] 
            StringQ@date]
         ],
        Button["Cancel", SetOptions[nb, Visible -> True]; 
         DialogReturn[], ImageSize -> Automatic], 
        ActionMenu[
         "?", {"Long text about who made this program code, when,\n\
what is the version number and how to use it..."}]}
       }}
    ],
    NotebookEventActions :> {"WindowClose" :> (SetOptions[nb, 
        Visible -> True])}
   ]
  ];

Myfunction is in the same notebook before the command nb = EvaluationNotebook[];:

Myfunction[pathFunctions_, pathInput_, date0_]:=
Module[
{string1=pathFunctions,string2=pathInput,AbsoluteDate0},
AbsoluteDate0=AbsoluteTime[{date0, {"Day", "Month", "Year"}}];
{string1,string2,AbsoluteDate0}
]

Consider the following. When clicking in "Browse" and then choosing a file, the path will remain the original one and won't change to the one of the file I chose:

nb = EvaluationNotebook[]; 

DynamicModule[
 {path1 = FileNameJoin[{$HomeDirectory, "Desktop", "file.nb"}],
  path2 = FileNameJoin[{$HomeDirectory, "Desktop", "file.xls"}]},  
 CreateDialog[
  Column[{Row[{InputField[Dynamic[path1], String, Enabled -> False], 
      FileNameSetter[Dynamic[path1], 
       "Open", {"Mathematica Notebook" -> {"*.nb"}}]}], 
    Row[{InputField[Dynamic[path2], String, Enabled -> False], 
      FileNameSetter[Dynamic[path2], 
       "Open", {"Excel files" -> {"*.xls", "*.xlsx"}}]}]}],  
  NotebookEventActions :> {"WindowClose" :> (SetOptions[nb, 
       Visible -> True])}]
]