Send an email notification to the manager and manager should assign the task to the users

The easiest way is to use the following actions:

• On item, send assign task to manager. 
• On task form, add a people picker control to collect person the task should be assigned to.
• This should push that information back to the task list
• Use the query list to grab and store that value of the people picker into a workflow variable 
• Reference that value in a new assign task to the person who needs the task. 

Of course it will take some tinkering but that is the general flow of it.