Sergey-

>I still don't understand while are you using !EOF("Table_B") instead of FOUND("TABLE_B")

SET RELATION TO Number INTO TABLE_B
SCAN FOR Amount<>0
  IF FOUND("TABLE_B")
      SELECT TABLE_B
      REPLACE Field2B WITH TABLE_A.Amount

If there's a relation from Table_A into Table_B and the there isn't a match in Table_B, then Table_B's record pointer will be at EOF().