Request payment part way through course

I'd like students to be able to access the first couple of lessons in Moodle before being presented with a request for payment. I've searched for ages on Google and found nothing, and also searched for all the possible terms I could think of here, and again come up short. Apologies therefore for the lack of contributing research/evidence. I'm using Paypal as the chosen method of payment at the moment. Moodle is version 3.2. Is there a way to add this kind of option to the 'access restrictions' in the courses themselves?