Marshalling a char** in C#

I am interfacing with code that takes a char** (that is, a pointer to a string):

int DoSomething(Whatever* handle, char** error);

Basically, it takes a handle to its state, and if something goes wrong, it returns an error code and optionally an error message (the memory is allocated externally and freed with a second function. That part I've figued out :) ).

I, however, am unsure how to handle in in C#. What I have currently:

[DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl)]
private static unsafe extern int DoSomething(IntPtr handle, byte** error);

public static unsafe int DoSomething(IntPtr handle, out string error) {
    byte* buff;

    int ret = DoSomething(handle, &buff);

    if(buff != 0) {
        // ???
    } else {
        error = "";
    }

    return ret;
}

I've poked around, but I can't figure out how to turn that into a byte[], suitable for feeding to UTF8Encoding.UTF8.GetString()

Am I on the right track?

EDIT: To make more explicit, the library function allocates memory, which must be freed by calling another library function. If a solution does not leave me with a pointer I can free, the solution is unacceptable.

Bonus question: As implied above, this library uses UTF-8 for its strings. Do I need to do anything special in my P/Invokes, or just use string for normal const char* parameters?