The viral donkey riddle. 90% CAN'T SOLVE IT

If you look at the following riddle, it seems clear and easy. But I can almost guarantee you, that your first answer will be wrong!

The first time the man bought a donkey for 50$, so he has an effective cash of -50$

He then sold it for 60$, which means that his effective cash is now +10$ (-50$ + 60$)

He buys the next donkey for 70$ bringing his effective cash down to -60$ (+10$ - 70$)

When he finally sells the second donkey, he get's back 80$, which brings his effective cash to 20$ (-60$ +80$)

In the end he made 20$.