Can't believe Ajax actually went back for him after his shenanigans last year with his agent, they don't strike me as a club quick to forgive.
Perhaps him being an Amsterdam kid swung it.
He's a very likeable guy. He has a good relationship with the people inside the club. I think it helped that he seemed to genuinely regret his choice quite quickly. Most of the time, these players are 'sorry' after 4 years, when their contract only runs for another year and then they suddenly are open to returning. Of course, then it's also very beneficial for them to get another long term contract. Brobbey was really keen on returning after 6 months (and probably acknowledged this even faster in private). Also after the season Brobbey was clear that he wanted a permanent move back. He still had 4 years left on his deal and I'm pretty sure he won't earn as much money back at Ajax as what Leipzig was paying him. Wouldn't even be suprised if Ajax made him pay a part of the fee either.
Ajax in general definitely won't / shouldn't be so forgiving, but this was just an exceptional case. On the other side Ajax also had an incentive because it needed a young striker for the long term. Obviously he is rated within the club. Therefore this all added up and Ajax was willing to be forgiving in this case.