How can i handle multiple data type for a same key from API in Flutter?

In the dart data model for offer_price field make its datatype as dynamic via which you’ll be able to have a dynamic data in it as run type. And while consuming this in any place just check the runtimeType of the variable and use it by type casting or just using it with .toString().

eg class:

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;
  }
}

For consuming it just try:

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)
           : ""