Is there a sin upon one who forgets what he studied with the intention of reviewing it later, and is that considered seeking knowledge?

Distance learning is like in-person learning in that it is a pursuit of knowledge. However, studying that relies on books without directly taking from scholars has pitfalls. Whoever does not acquire knowledge from a shaykh is feared to fall into many errors. As for forgetfulness, there is no blame for it, but negligence and causing it is not the habit of those with high aspirations. So, we advise frequent repetition and supplication. The way to know sincerity is to seek knowledge for the sake of Allah and with a desire to invite others to Islam. The duty towards knowledge is to act upon it, invite others to it, and disseminate it. The money earned from teaching what you know is lawful.