If you'd like to allow your learners to access your paid course free of charge, here's how:

 You can create a separate, free-taster course. This would usually contain a small portion of your original course, perhaps the first module only while hinting at what's comes next - in the paid full version.

When setting up a free-taster course, other elements to consider include: how you are nurturing the learner through to the paid course, whether you've provided contact details or activities in the taster course so that they can ask you any questions they may have before enrolling, and whether you want to offer a discount if they enroll in the paid version within a particular timeframe. 

 Alternative: Enrol the learner via Administer learners (that will allow them to bypass the payment) in the course and remove them manually after a certain number of days. If they attempt to access the course again after, they will be prompted to pay. After purchasing the course, they will be able to continue where they previously stopped. (they won't lose their progress)

Related: 

How do I set up a discount?

How do I copy a course?
How do I copy a page from one course to another?