I know this has been covered before, but I've had a similar problem recently, with a strange twist.
Last week, a user created a credit memo, and for whatever reason, a record got put in the SY00500 table with their UserID as the batch ID, and they were getting the "Your previous transaction-level posting...." message. I followed the previously described processes and all was well for a few days.
Now, today, he told me he was getting that same "Your previous transaction-level...." message again. I checked the SY00500 table and again, a record was sitting out there with his UserID in the batch ID field. However, the weird part was the batch total amount on the record had nothing to do with what he was working on today - instead, it's the amount from the record I deleted last week! If it matters, the BCHSOURC field is XRM_Sales, the RCLPSTDT is today, and the BCHTOTAL value is $828.00 - the amount he posted LAST week (not today's amount).
Where can this mysterious record be coming from??