[Vuejs]-Vue components: props undefinded

in LadevorgangAendern.vue, try

<p>Preis: {{ this.preis }}</p>

Perhaps try including vuex state management in your application. Depending on how often or where you want to access your data, its a great way to create a single state throughout the app without having to continually pass props down or emit them back up or using an event bus.

https://vuex.vuejs.org/