What is the ruling on participating in an online training course if a refundable deposit is required, from which 30% is deducted as administrative fees after the courses conclude?

There is no harm in subscribing with this condition, because the subscription fee fluctuates between two known amounts (500 pounds for those who did not complete the course, and 150 pounds for those who did). This does not involve any ambiguity. It is permissible for the fee to fluctuate between two known values that relate to two precise descriptions, such as linking it to the time of completing the work, or to two places, or to two different types; because this does not lead to gharar (excessive uncertainty) or jahalah (ignorance), and it has become customary and has been permitted by some jurists.