P4310 Clearing Amounts

Users are entering lump sum purchase order detail lines. Once these lines are created, P4310 will display a hard error (#3289) if a qty and unit rate are added. On the other hand, should the user tab through the lump sum grid line, the extended cost will be recalculated to zero (0x0=0).  How do you prevent the clearing of the order amount is P4310 won't permit changes to the lump sum type of order?