As everyone knows, negative tiers happen because of timing issues in processing. Since the controller isn't into third party enhancements, then the controller should start investigating why this is happening and implement procedures to prevent it. It is all about timing. All PO receipts are processed and updated by 10 AM, All shop floor transactions are processed by 2 PM, work orders are completed by 4 PM, SO invoices are done by 5 PM. Wa la, no more negative tiers. Failure to adher to these posting deadlines will be punishable by death or worse, a job at Sage..................
Run the negative tier report on a daily basis to determine where processing delay is occurring and deal with it. This is how I resolved a lot of the issues back in the day when I used MAS (sota) in real life................