I"m guessing this is the U.S.?
Here's how it goes. It takes an average of seven days to clear a check in the United States. However, U.S. law requires that banks allow access to the funds within five days.
Therefore, there is a two day 'float' period in which he has 'free money'.
The scammer will then request that he sends a portion of it back: usually 50%
scammer --> $20,000 check -->victim --> bank --> float --> victim --> $10,000 wire --> scammer
Now, after, the two day float period has ended; the check will bounce, the transaction is reversed and the victim is left with a -$10,000 bank balance.
Don't fall for that shit. Even if the check clears it will be reversed.