The answer is very simple; you have to speculate to accumulate. As a very young man, Bischoff was a highly rated talent, something that Wenger was well aware of. Shortly after signing for Werder Bremen he received a career threatening injury and the club didn’t know what to do with him. Wenger leapt on the opportunity to pick up such a potential talent for free. Upon signing him, he, very honestly in my opinion, announced in a press statement that the signing was a gamble. A gamble that, as far as I can see, had three possible outcomes:
1 The most ideal would have been that Bischoff turned out to be a very good player and a valuable asset to the squad.
2 Next in order of importance, he could have been good enough to sell at a healthy profit. It is easily forgotten that Arsenal view their youth system as an important income stream and any profit made continues to finance the system.
3 And this seems to me to be what is happening, having not made the grade Arsenal are helping him get another club more suitable to his level.
You may ask, what was the point? To which, my guess would be this: many potentially great players get unfortunate injuries at a young age and are discarded by their clubs. By helping Bischoff, Arsenal sent a message to all the fathers of these unfortunate players that Arsenal is a club that will do the right thing if they take a risk with one of them. Bischoff has turned out to be a waste of time but the next one might be a Maradonna, ridiculous I hear you cry to which I would say once again:
You have to speculate to accumulate.