Handle Format exception in Decimal

Hi Experts,
I created a property 
public decimal? Amount{ get; set; } When a user enter any string value, it gives an error of Format Exception, since it is decimal
I am trying to handle it using following code
Amount = IIf(Decimal.TryParse(cov.Amount.ToString().Replace("$", ""), a), Convert.ToDecimal(cov.Amount.ToString().Replace("$", "")), a) But it is not solving my issue.
Please help.
Thanks, Rahul