Decimal Null value on Form Post

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MVC

C#5.0

Requirement : Show the Numbers in Currency format.
e.g: 12,13,4575.54

So in Form view is use "{0:C}"
see below

@using (Html.BeginForm("PuchaseOrder", "PurchaseOrder", FormMethod.Post, new  { @id = "PO_Form" }))
      {
       @Html.TextBoxFor(model => model.Purchase_Amount_AED,"{0:C}", new { @class = "text-field validate[required]" })
    }

When i am submitting the form, other values are posting but this value is null.

[HttpPost]
        public ActionResult PuchaseOrder(PO_PuchaseOrderModel REC)
        {
            decimal? Rate_USD_TO_AED = REC.Purchase_Amount_AED;
        }

How can I resolve this problem.
Thanks