How could he have possibly known how much was in the account? He was in jail. No internet there..
The consensus from the articles I've read is that Zimmerman and his wife talked in code while he was in jail, and they both knew about it. From this URL:
State prosecutors said on Friday Zimmerman's wife, Shelley, knew about the PayPal money from his website and lied at the bond hearing when she said she didn't know of it.