Mock app.config value

public class FundsTransferAuthorization
    {
        public decimal MaxAmount { get; set; }
        public decimal MinAmount { get; set; }
        
        private readonly IPromptPlayer _promptPlayer;
        public FundsTransferAuthorization(IPromptPlayer promptPlayer)
        {
            _promptPlayer = promptPlayer;
            GetAuthorizationParameters();
        }

        private void GetAuthorizationParameters()
        {
            _maxDigit = _promptPlayer.MaxDigit;
             MaxAmount = Convert.ToDecimal(GetMaximumAmount());
             MinAmount = Convert.ToDecimal(GetMinimumAmount());
        }
        public virtual string GetMinimumAmount()
        {
            return ConfigurationManager.AppSettings["minimumAmount"];
        }
        public virtual string GetMaximumAmount()
        {
            return ConfigurationManager.AppSettings["maximumAmount"];
        }