If Customer and Sales are locked to one another, you may want a relation that is Customer>>Sales>>Surveys.
The setup you have now allows Customers and Sales to have the many-to-many relationship by way of the survey table. The Customer records can have many different Sales associations, and Sales can have many different Customer associations.
If selecting a Customer in the Survey record should also cause a particular Sales record to be brought in as a lookup, then it seems, you're wanting the top down relationship hierarchy where Customer has many Sales, and Sales has many surveys.
Let's see what others may say.