The simplest way I can think of is making one course into two.
Course 1 has topics 1 - 10 and is free.
Course 2 topics 11 -25 are the extension of course 1 and has the Paypal enrolment on it.
The next obvious question is how to make enroling in course 2 dependant on results or completion of course 1?
A simple but not so clean solution is to only allow course two to be enrolable in say the last week of course one. That you can do in Paypal.