I'm learning drupal, I create a web form for duty travel that the user submitted. Now I want to create a workflow so after submitted the webform should be sent to manager for validation or cancelling after that he add some comments and files and resend it to the user
You can setup complete workflow using these modules with webform:
This is a Drupal 7 module which allows assigning workflow 'states' to webform submissions. Users can be notified when submissions change state. Permissions can be set to control access to submissions based on their state. The states themselves are fieldable Drupal entities.
workflows are made up of workflow states. Transitions between states can be allowed per role. For example, a workflow with the states Draft, Review, and Published could be assigned to the Story node type. Only users with role 'chief editor' can set Stories to the published state. You can set up the Workflow to alter states from form, page, comment, a special block, and a special workflow tab. Additionally, the submodule Workflow Access allows you to manage view/edit/delete permissions for content types per User role and Workflow state.
Transitions between workflow states can have actions assigned to them. In our example, we could assign an action so that when the story moves from the Draft state to the Review state an email is sent out. Another action could be assigned to the transition from Review to Published so that the node's status is set to Published
You can use hooks to change almost anything in the webform like additional submit handlers can be handled, validation can be done, elements can be added, etc.
Refer the already existing answers, it will be helpful.