MVVM and async properties

I have a ViewModel with the following property

public Employee SelectedEmployee
        {
            get { return _selectedEmployee; }
            set
            {
                if (value == _selectedEmployee) return;
                _selectedEmployee = value;

                if (_selectedEmployee != null)
                {
                    StaffHolidaysViewModel.HolidayAllowance = _staffDataService.EmployeeHolidayAllowance(_selectedEmployee.Id);
                    FireEmployeeSelectedMessage(SelectedEmployee.Id);
                }

                RaisePropertyChanged();
                RaisePropertyChanged(nameof(Allowance));
                RaisePropertyChanged(nameof(Taken));
                RaisePropertyChanged(nameof(Remaining));
                RaisePropertyChanged(nameof(TotalAbsences));
                RaisePropertyChanged(nameof(TotalSick));
                RaisePropertyChanged(nameof(TotalNonSickAbsences));
                RaisePropertyChanged(nameof(SelectedEmployeeLeavingDate));
                UpdateCanExecuteChanged();
            }
        }

The line

StaffHolidaysViewModel.HolidayAllowance = _staffDataService.EmployeeHolidayAllowance(_selectedEmployee.Id);

Contains a call to a method that I'd like to make asynchronous. I don't know how I can do this as SelectedEmployee is a bound property.

<ComboBox Name="StaffMembers" ItemsSource="{Binding FilteredEmployees}" SelectedItem="{Binding SelectedEmployee}" Width="200" BorderThickness="1" BorderBrush="DimGray">
                    <ComboBox.ItemTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding Name}"/>
                        </DataTemplate>
                    </ComboBox.ItemTemplate>
                </ComboBox>

Since it's a property, I can't make its type Task<Employee>

What's the recommended approach to async bound MVVM properties? Or is it just not possible?