From the approach you are taking, I would try working with a customized Error handling routine to try the following. Although brief, I hope to open the concept for you to expand upon...

llOk = .T.
lcOldErrorRoutine = on( "error" )
begin transaction
llSuccess = TableUpdate()
if not llSuccess
  aerror( laError )
  if laError = 1884
    */ Uniqueness error number
    */ RE-Generate new number...
    llOk = .F.
  endif
endif
rollback

on error &lcOldErrorRoutine
return( llOk )

You may or may not want to rollback depending on when the attempt is being checked... I would try it during a loop when determining the next available number... Then check again when user tries to save the changes.