Try something like this:

Regex negativeValueRegex = new Regex(@"\-[0-9]*.[0-9]*");

var grid = (Sage.ObjectStore.Controls.Grid)
    _form.FindControlByName("postedTransGrid").UnderlyingControl;

grid.Format += (s, e) =>
{
    if(negativeValueRegex.IsMatch( e.Value.ToString() ))
    {
        e.Cancel = true;
        e.Value = $"(-){e.Value.ToString().Substring(1)}";
    }
};

thank you that works..

I just add few things to add separator

Regex negativeValueRegex = new Regex(@"\-[0-9]*.[0-9]*");
postedTransGrid.Format += (s, e) =>
{
decimal number;
if (negativeValueRegex.IsMatch(e.Value.ToString()))
{
e.Cancel = true;
e.Value = $"(-){Convert.ToDecimal(e.Value).ToString("N", new CultureInfo("en-US")).Substring(1)}";
}
else if(Decimal.TryParse(e.Value.ToString(), out number))
{
e.Cancel = true;
e.Value = $"{Convert.ToDecimal(e.Value).ToString("N", new CultureInfo("en-US"))}";
}
};