Model

public partial class OrderDetail : Entity {

    [DataMember]
    public string UnitPriceFormatted {
      get {
        return PropertyMetadata.UnitPriceFormatted.GetValue(this);
      }
      set {
        PropertyMetadata.UnitPriceFormatted.SetValue(this, value);
        bool error = this.EntityAspect.ValidationErrors.Any(vr => vr.TriggerMemberName == PropertyMetadata.UnitPriceFormatted.Name);
        if (!error) {
          var decimalValue = Convert.ToDecimal(value);
          UnitPrice = decimalValue;
        }
      }
    }

    public partial class PropertyMetadata {
      public static readonly DataEntityProperty<OrderDetail, string> UnitPriceFormatted =
        new DataEntityProperty<OrderDetail, string>("UnitPriceFormatted", true, false, ConcurrencyStrategy.None, false, false);
    }

DevForce Validation

    public class VerifierProvider : IVerifierProvider {

      public IEnumerable<Verifier> GetVerifiers(object verifierProviderContext) {
        List<Verifier> verifiers = new List<Verifier>();
        verifiers.Add(GetDecimalRangeFormattedVerifier());

        return verifiers;
      }
    }

    private static Verifier GetDecimalRangeFormattedVerifier() {
      Verifier v = new DecimalRangeFormattedVerifier(typeof(OrderDetail), PropertyMetadata.UnitPriceFormatted.Name);
      v.ErrorMessageInfo.ErrorMessage = "You have entered an invalid decimal number";

      return v;
    }

    public class DecimalRangeFormattedVerifier : PropertyValueVerifier {

      public DecimalRangeFormattedVerifier(Type pType, String propertyName, String displayName = null,
        bool? shouldTreatEmptyStringAsNull = null)
        : base(new PropertyValueVerifierArgs(pType, propertyName, false, displayName, shouldTreatEmptyStringAsNull)) {
      }

      public DecimalRangeFormattedVerifier(PropertyValueVerifierArgs args) : base(args) { }

      protected override VerifierResult VerifyValue(object itemToVerify, object valueToVerify,
        TriggerContext triggerContext, VerifierContext verifierContext) {

        var aString = (string)valueToVerify;
        decimal result;
        var isDecValueOk = Decimal.TryParse(aString, out result);
        VerifierResult vr = new VerifierResult(isDecValueOk);

        return vr;
      }
    }

Employee Model

public partial class Employee : Entity {

    [DecimalRangeVerifier(MinValue = 0d, MaxValue = 100d)]
    [DataMember]
    public string PaymentFormatted {
      get {
        return PropertyMetadata.Payment.GetValue(this);
      }
      set {
        PropertyMetadata.Payment.SetValue(this, value);
      }
    }

    public partial class PropertyMetadata {
      public static readonly DataEntityProperty<Employee, string> Payment =
        new DataEntityProperty<Employee, string>
                                          ("PaymentFormatted", true, false, ConcurrencyStrategy.None, false,   false);
    }

}

ViewModel

    _mainMgr.Employees.Take(1).ExecuteAsync(op => {
        AnEmployee = op.Results.Cast<Employee>().FirstOrDefault();
        AnEmployee.PaymentFormatted = "0";
      });

    public Employee AnEmployee {
      get {
        return _anEmployee;
      }
      set {
        _anEmployee = value;
        RaisePropertyChanged("AnEmployee");
      }
    }

View (xaml)

<StackPanel Grid.Row="3" DataContext="{Binding AnEmployee}">
            <TextBox Name="txtOutput"
                 Text="{Binding PaymentFormatted, Mode=TwoWay, NotifyOnValidationError=True,
                                  FallbackValue='Binding has failed'}"
                 Height="25" Width="150"
                 VerticalScrollBarVisibility="Auto" />

</StackPanel>