Permissibility of having different fees for an online course

Q. I want to enroll for an on-line course. The on-line course offers a discount on advance payments of fess while it offers a payment of normal monthly fees. Is it permissible to have two different fess for advance payments of fees and normal monthly fees in Shariah?

A. It is permissible to have a discounted fee for advance payments of fees and normal fees for monthly payments on condition that the fees are specified and does not fluctuate. (Badaai-us-Sanaai)