function bb_add_stock_reducer_single_product_field() {
	?><div class='options_group'><?php
		woocommerce_wp_text_input( array(
			'id'				=> '_stock_reducer',
			'label'				=> __( 'Inventory reduction per item sold', 'woocommerce' ),
			'placeholder'			=> __( 'When purchased, reduce stock level with this much', 'woocommerce' ),
			'type' 				=> 'number',
			'custom_attributes'		=> array(
				'min'	=> '1',
				'step'	=> '1',
			),
		) );
	?></div><?php
}
add_action( 'woocommerce_product_options_inventory_product_data', 'bb_add_stock_reducer_single_product_field' );


function bb_add_stock_reducer_variable_product_field( $loop, $variation_data, $variation ) {
	$variation = wc_get_product( $variation );
	woocommerce_wp_text_input( array(
		'id'				=> "stock_multiplier{$loop}",
		'name'				=> "stock_multiplier[{$loop}]",
		'value'				=> $variation->get_meta( '_stock_reducer' ),
		'label'				=> __( 'Inventory reduction per variable item sold', 'woocommerce' ),
		'placeholder'			=> __( 'When purchased, reduce stock level with this much', 'woocommerce' ),
		'type' 				=> 'number',
		'custom_attributes'	=> array(
			'min'	=> '1',
			'step'	=> '1',
		),
	) );
}
add_action( 'woocommerce_variation_options_pricing', 'bb_add_stock_reducer_variable_product_field', 50, 3 );


function bb_save_single_product_stock_reducer( $product ) {
	if ( ! empty( $_POST['_stock_reducer'] ) ) {
		$product->update_meta_data( '_stock_reducer', absint( $_POST['_stock_reducer'] ) );
	}
}
add_action( 'woocommerce_admin_process_product_object', 'bb_save_single_product_stock_reducer'  );


function bb_save_variable_product_stock_reducer( $variation_id, $i ) {
    $variation = wc_get_product( $variation_id );
	if ( ! empty( $_POST['stock_multiplier'] ) && ! empty( $_POST['stock_multiplier'][ $i ] ) ) {
		$variation->update_meta_data( '_stock_reducer', absint( $_POST['stock_multiplier'][ $i ] ) );
		$variation->save();
	}
}
add_action( 'woocommerce_save_product_variation', 'bb_save_variable_product_stock_reducer', 10, 2 );


function bb_stock_reducer( $quantity, $order, $item ) {
	$multiplier = $item->get_product()->get_meta( '_stock_reducer' );
	if ( empty( $multiplier ) && $item->get_product()->is_type( 'variation' ) ) {
		$product = wc_get_product( $item->get_product()->get_parent_id() );
		$multiplier = $product->get_meta( '_stock_reducer' );
	}
	if ( ! empty( $multiplier ) ) {
		$quantity = $multiplier * $quantity;
	}
	return $quantity;
}
add_filter( 'woocommerce_order_item_quantity', 'bb_stock_reducer', 10, 3 );