Swapping values between columns suddenly stopped working

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C#

MySQL

Hi All, I have an unusual problem. My code worked fine for months and now all of a sudden it gives me an error and I did not change a thing in this area. The code swaps two column values when a user presses a button. This is simple right? well it does not work anymore. I don't know what went wrong. I thought it might be that I updated my MySQL database, but when I try to use the syntax in MySQL itself, it is working. So the problem is with my code..

This is my code:

public void UpdatePayment(int paymentId)
        {
            using (MySqlConnection Conn = new MySqlConnection(Connect.sConnStr))
            {
                Conn.Open();
                string sSql = "UPDATE payments SET amount=(@temp:=amount), amount = owes, owes = @temp WHERE payment_id = " + paymentId + ";";
                int iReturn = Biogym.Service.Classes.Connect.getDataCommand(sSql, Conn).ExecuteNonQuery();
                Conn.Close();
                Conn.Dispose();
            }
        }

What is wrong?? It suddenly says "Parameter @temp is not defined", but I mean how can it work for months and suddenly stop??