I'm trying to overwrite Orderline price in POS Odoo

**My price.js**

    get_unit_display_price: function(){    

        var self = this;           

        var line = self.export_as_JSON();
        var product = this.pos.db.get_product_by_id(line.product_id);     

        fields.product_id  = line.product_id;
        fields.pricelist_id   = this.pos.config.pricelist_id[0];
        fields.uom = product.uom_id;
        fields.line_qty = line.qty;
        fields.price_unit = line.price_unit;
        var model = new Model('pos.order');
         this.total_price = model.call('calculate_price',         

                  [0, fields]).done(function(result){       

              total_price = result['total_price'];
                     return  result['total_price'];
                });
             }

**price.xml**

            <t t-jquery=".price" t-operation="append">              

           <t t-esc="widget.format_currency(line.get_unit_display_price)"/>     

            </t>

I'm getting value **total_price** from Model (price.py)

But returning is **undefined** in get_unit_display_price in xml file.
How to set value in xml from js after execution of new model function (js value from model) ?.