Install module with required config that may exist

I have a module that have some config in config/install folder, that require config that may already exist in the current config. So I placed those config under config/optional.

But if those config does not exist, they are not installed and the module installation throw an error for unmet dependancies.

What can I do to have a module that have dependancies on config that may exist ?

Thanks.

More information:

Under config/install : core.entity_view_display.user.user.teaser_resource.yml

langcode: fr
status: true
dependencies:
  config:
    - core.entity_view_mode.user.teaser_resource
    - field.field.user.user.field_firstname
    - field.field.user.user.user_picture
  module:
    - user
id: user.user.teaser_resource
targetEntityType: user
bundle: user
mode: teaser_resource
content:
  field_firstname:
    type: string
    label: hidden
    settings:
      link_to_entity: false
    third_party_settings: {  }
    weight: 0
    region: content
hidden:
  langcode: true
  member_for: true
  realname: true
  search_api_excerpt: true
  user_picture: true

Under config/optional: field.field.user.user.field_firstname.yml

langcode: fr
status: true
dependencies:
  config:
    - field.storage.user.field_firstname
  module:
    - user
id: user.user.field_firstname
field_name: field_firstname
entity_type: user
bundle: user
label: Prénom
description: ''
required: true
translatable: false
default_value: {  }
default_value_callback: ''
settings: {  }
field_type: string

(field.storage.user.field_firstname.yml is also there)

The error is :

Unable to install Custom Module due to unmet dependencies: core.entity_view_display.user.user.teaser_resource (field.field.user.user.field_firstname)