This is really unprofessional, and the fact you ask for 50% of the payment to be upfront you really are tarnishing whatever reputation you had built up. I think you should fully refund slinky of his money, doesn’t matter if you finished it or what not the fact that you’re busy I understand, but it was said you were changing discord profile pictures which means you had sometime.
Even if you were busy everyday and you didn’t change your discord profile, it literally takes less then 5 minutes to maybe go discord and send a message to slinky saying, I can’t get it done by this date, you weren’t clear enough, etc. The fact that you haven’t responded to him on discord for 5 days, and that he paid you upfront is horrible.
For whatever reason this occurred I hope its not the fact that just because slinky isn’t a massive dev or someone with huge connections and reputation he was taken advantage of like this. I hope you take responsibility for this mistake, the fact that you are 20 and did this shows immaturity and how unprofessional you are. Also along with the fact you literally stated you have lots of free time multiple times in the OP, I realize its from February but you could easily edit and put that you don’t have as much time, simple.