you are right .. but this would be a hassle for the admin .. he would need to set it back every time ..
have the payout cronjob starting at 5 am .. and the slave creates a dump at 4.59 am .. this would be the only way to cover the risk of temporary manipulation ..
another solution would be a monitoring script that creates a temp database that creates a snapshot x hours before payout and compares it with the table after payout ..
btw, if you use MySql for replication be sure to update to 4.0.20 ..
this bug caused me and many others a shitload of time :
http://bugs.mysql.com/bug.php?id=2886