DropDownList has a SelectedValue which is invalid because it does not exist in the list of items. Parameter name: value

Attempt to find the value in the drop down list before attempting to set the SelectedValue, like this:

if (DropDownListCurrency.Items.FindByValue(row.Cells[8].Text.ToString().Trim()) != null)
{
    DropDownListCurrency.SelectedValue = row.Cells[8].Text.ToString().Trim();
}

Note: The Trim() call will remove any leading or trailing spaces in your text box text, which could be a cause for a match not being found.

So your full code should be this:

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
    try 
    {
        GridViewRow row = GridView1.SelectedRow; 

        AccountNumber.Text = (string)row.Cells[0].Text;
         ....

        if (DropDownListCurrency.Items.FindByValue(row.Cells[8].Text.ToString().Trim()) != null)
        {
            DropDownListCurrency.SelectedValue = row.Cells[8].Text.ToString().Trim();
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine("{0} Exception caught.", ex);
    }
}

it's so simple just use this

Dropdown.SelectedValue = null;
Dropdown.DataBind();

Hi maybe in that cell from your gridview have white space or dropdownlist have white space for example isn't the same this

Dolar__ = Dolar

or

Dolar = Dolar__

use a Trim in code behind to clear white spaces in SQL Server don't use Rtrim this isn't good practices