Assigning content from one user to another user automatically after a payment
Use the Node access user reference module for this purpose. About this module (quote from its project page):

Gives content access permissions to users for content that references the users with User reference or Entity reference.
This is great if you want your content authors to be able to choose additional authors, or choose who can view their content, as well as being able to display the list of authors or viewers within your content, or in views displays.

And use the Entity reference module to implement your &quot;assignments&quot;.
To actually &quot;use&quot; (configure) these settings, refer to the module's README.txt, which includes these instructions:

Create a field to reference users in a content type using the Field UI.  The field's configuration page will contain the settings for Node access user reference. For detailed instructions on using the Field UI see:
https://www.drupal.org/documentation/modules/field-ui

Possibly you also might want to use the Field Permissions module.