Error on a textbox - Input string not in a correct format

namespace AfricanFoodsPOS
{
    public partial class Payment : Form
    {
        private BindingList<tblproduct> products = new BindingList<tblproduct>();

        AFStagingEntities afNewObject = new AFStagingEntities();

        public delegate void PaymentMadeEvent(object sender, PaymentMadeEventArgs e);

        //public decimal myValue = 0;

        public event PaymentMadeEvent PaymentMade;

        public decimal myPay;
        public decimal MyPay
        {
            get { return myPay; }
            set {
                  myPay = value;
                  txtYouArePaying.Text = String.Format("{0:c}", myPay);                 
                } 
        }

        public decimal paymentAmount;
        public decimal PaymentAmount
        {
            get { return paymentAmount; }
            set
            { 
                paymentAmount = value;             
                txtAmountToPay.Text = String.Format("{0:c}", paymentAmount);                
            }
        }       

        public Payment()
        {
            InitializeComponent();         
              
        }


        //was private
        public void PaymentHasBeenMade(object sender, EventArgs e)
        {
            decimal total = 0;           
            //robust
            try
            {
                decimal YouArePaying = 0;
                decimal AmountToPay = 0;
                YouArePaying = decimal.Parse(txtYouArePaying.Text.TrimStart('R'));
                AmountToPay = decimal.Parse(txtAmountToPay.Text.TrimStart('R'));

                total = YouArePaying - AmountToPay;

                if (total >= 0)
                {
                    MessageBox.Show("Give change: R " + total);
                    PaymentMade(this, new PaymentMadeEventArgs() { PaymentSuccess = true });
                }
                else
                {
                    total = AmountToPay - YouArePaying;
                    txtAmountToPay.Text = total.ToString();

                    MessageBox.Show("Customer is: R " + total + " short.");
                    PaymentMade(this, new PaymentMadeEventArgs() { PaymentSuccess = false });
                }
            }
            catch (Exception exx)
            {
                MessageBox.Show("An error has occured, please enter a valid amount, " + exx.Message);
            }

            finally
            {
                //txtYouArePaying.Dispose();          
            }
        }


        private void Payment_Load(object sender, EventArgs e)
        {

        }     

    }


    public class PaymentMadeEventArgs : EventArgs
    {
        private bool paymentSuccess;

        public bool PaymentSuccess
        {
            get { return paymentSuccess; }
            set { paymentSuccess = value; }
        }
    }

    
}