C# Using span with SocketAsyncEventArgs

As already mentioned in the comments, Span is the wrong tool here - have you looked at using Memory instead? As you stated, the SetBuffer method does accept that as a parameter - is there a reason you can't use it?

See also this article for a good explanation on how stack vs heap allocation applies to Span and Memory. It includes this example, using a readonly Memory<Foo> buffer:

public struct Enumerable : IEnumerable<Foo>
{
    readonly Stream stream;

    public Enumerable(Stream stream)
    {
        this.stream = stream;
    }

    public IEnumerator<Foo> GetEnumerator() => new Enumerator(this);

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

    public struct Enumerator : IEnumerator<Foo>
    {
        static readonly int ItemSize = Unsafe.SizeOf<Foo>();

        readonly Stream stream;
        readonly Memory<Foo> buffer;
        bool lastBuffer;
        long loadedItems;
        int currentItem;

        public Enumerator(Enumerable enumerable)
        {
            stream = enumerable.stream;
            buffer = new Foo[100]; // alloc items buffer
            lastBuffer = false;
            loadedItems = 0;
            currentItem = -1;
        }

        public Foo Current => buffer.Span[currentItem];

        object IEnumerator.Current => Current;

        public bool MoveNext()
        {
            if (++currentItem != loadedItems) // increment current position and check if reached end of buffer
                return true;
            if (lastBuffer) // check if it was the last buffer
                return false;

            // get next buffer
            var rawBuffer = MemoryMarshal.Cast<Foo, byte>(buffer);
            var bytesRead = stream.Read(rawBuffer);
            lastBuffer = bytesRead < rawBuffer.Length;
            currentItem = 0;
            loadedItems = bytesRead / ItemSize;
            return loadedItems != 0;
        }

        public void Reset() => throw new NotImplementedException();

        public void Dispose()
        {
            // nothing to do
        }
    }
}

You should copy the data to managed memory first, use Marshal or Buffer class.

If not think about when the C code delete the returned pointer what will happen to the send data?

There's a complete example (and implementation of the class) on the MSDN page for the SocketAsyncEventArgs (just follow the link). It shows the proper use of the class and may give you the guidence you're looking for.

Also, as Shingo said, it should all be in managed code, not in pointers.