class Product {
  int? id;
  int? price;

  // if you need the value as String
  String? offerPriceAsString;

  // Value as a double
  double? offerPrice;

  Product({this.id, this.price, this.offerPrice});

  Product.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    price = json['price'];
    double.parse("1.0");
    // Any value to String
    offerPriceAsString = json['offer_price'].toString();
    // String to Double
    offerPrice = double.parse(json['offer_price'].toString());
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    data['id'] = id;
    data['price'] = price;
    data['offer_price'] = offerPrice;
    return data;
  }
}

在dart数据模型的offer_price字段中，将其数据类型设置为dynamic，这样就可以将动态数据设置为run类型。在任何地方使用此变量时，只需检查变量的runtimeType，并通过类型转换或仅将其用于.toString()。
例如班级：

class Products {
  Products({this.product});

  Products.fromJson(Map<String, dynamic> json) {
    if (json['product'] != null) {
      product = <Product>[];
      json['product'].forEach((v) {
        product!.add(Product.fromJson(v));
      });
    }
  }
  
  List<Product>? product;
  
  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    if (this.product != null) {
      data['product'] = this.product!.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Product {
  Product({this.id, this.price, this.offerPrice});
  
  Product.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    price = json['price'];
    offerPrice = json['offer_price'];
  }
  
  int? id;
  int? price;
  dynamic? offerPrice;

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    data['id'] = this.id;
    data['price'] = this.price;
    data['offer_price'] = this.offerPrice;
    return data;
  }
}

对于消费它只是尝试：

Products products = List<Products>[];

//assign products = Products.fromJson(jsonData);

//using
products[i].offerPrice == null
? "null"
: products[i].offerPrice.runtimeType == String
   ? products[i].offerPrice.toString()
   : products[i].offerPrice.runtimeType == Int
      ? int.parse(products[i].offerPrice)
      : products[i].offerPrice.runtimeType == Double
           ? double.parse(products[i].offerPrice)
           : ""