Tiptronic issue

Hey everyone. I'm new to the forum. Anyways, I'm selling my 98 1.8 turbo GLS Passat. The tiptronic function has stopped working about 6 months ago. The transmission runs fine when in normal automatic mode, when I slide over into tiptronic nothing happens. The potential buyer is held up on this issue . I think that its due to some electrical connection, and not the transmission. He just wants some assurance that the transmission is fine. So has anyone experienced this type of issue and if so what was the problem? I plan on taking it in sometime soon to see what the problem is. I hope I can get some help here first.

thanks,
Omran