Program: 17176
Project: 17176
Buggy source:
private Language buy(Player player){
  double price=getBuyPrice(sign.getLine(PRICE_LINE));
  if (price == NO_PRICE) {
    return NO_BUYING_HERE;
  }
  if (container == null) {
    return NO_CHEST_DETECTED;
  }
  if (!hasPermission(player,stock.getType(),true)) {
    return NO_PERMISSION;
  }
  String playerName=player.getName();
  String itemName=StringUtil.capitalizeFirstLetter(stock.getType().name());
  double balance=Economy.balance(playerName);
  if (!Economy.hasEnough(playerName,price)) {
    int possiblePartialItemCount=calculateItemAmount(balance,price);
    if (!partialTransactionAllowed(possiblePartialItemCount)) {
      return NOT_ENOUGH_MONEY;
    }
 else {
      price=(price / stock.getAmount()) * possiblePartialItemCount;
      stock.setAmount(possiblePartialItemCount);
    }
  }
  if (!stockFitsPlayer(player)) {
    return NOT_ENOUGH_SPACE_IN_INVENTORY;
  }
  if (!shopHasEnoughItems()) {
    int possiblePartialItemCount=getStockAmount(stock);
    if (!partialTransactionAllowed(possiblePartialItemCount)) {
      if (Config.getBoolean(SHOW_MESSAGE_OUT_OF_STOCK)) {
        sendMessageToOwner(Config.getLocal(NOT_ENOUGH_STOCK_IN_YOUR_SHOP).replace("String_Node_Str",itemName));
      }
      return NOT_ENOUGH_STOCK;
    }
 else {
      price=(price / stock.getAmount()) * possiblePartialItemCount;
      stock.setAmount(possiblePartialItemCount);
    }
  }
  Economy.add(getOwnerAccount(),price);
  Economy.subtract(playerName,price);
  container.removeItem(stock);
  InventoryUtil.add(stock,player.getInventory());
  player.updateInventory();
  TransactionEvent event=new TransactionEvent(BUY,container,sign,player,this.owner,stock,price);
  ChestShop.callEvent(event);
  return null;
}

Fixed version (ground truth):
private Language buy(Player player){
  double price=getBuyPrice(sign.getLine(PRICE_LINE));
  if (price == NO_PRICE) {
    return NO_BUYING_HERE;
  }
  if (container == null) {
    return NO_CHEST_DETECTED;
  }
  if (!hasPermission(player,stock.getType(),true)) {
    return NO_PERMISSION;
  }
  String playerName=player.getName();
  String itemName=StringUtil.capitalizeFirstLetter(stock.getType().name());
  double balance=Economy.balance(playerName);
  if (!Economy.hasEnough(playerName,price)) {
    int possiblePartialItemCount=calculateItemAmount(balance,price);
    if (!partialTransactionAllowed(possiblePartialItemCount)) {
      return NOT_ENOUGH_MONEY;
    }
 else {
      price=(price / stock.getAmount()) * possiblePartialItemCount;
      stock.setAmount(possiblePartialItemCount);
    }
  }
  if (!stockFitsPlayer(player)) {
    return NOT_ENOUGH_SPACE_IN_INVENTORY;
  }
  if (!shopHasEnoughItems()) {
    int possiblePartialItemCount=getStockAmount(stock);
    if (!partialTransactionAllowed(possiblePartialItemCount)) {
      if (Config.getBoolean(SHOW_MESSAGE_OUT_OF_STOCK)) {
        sendMessageToOwner(Config.getLocal(NOT_ENOUGH_STOCK_IN_YOUR_SHOP).replace("String_Node_Str",itemName));
      }
      return NOT_ENOUGH_STOCK;
    }
 else {
      price=(price / stock.getAmount()) * possiblePartialItemCount;
      stock.setAmount(possiblePartialItemCount);
    }
  }
  Economy.add(getOwnerAccount(),price);
  Economy.subtract(playerName,price);
  container.removeItem(stock.clone());
  InventoryUtil.add(stock.clone(),player.getInventory());
  player.updateInventory();
  TransactionEvent event=new TransactionEvent(BUY,container,sign,player,this.owner,stock,price);
  ChestShop.callEvent(event);
  return null;
}