Hi - the only real option you have in this case would be to add a separate "Cash" position to your portfolio and note that the dollar amount is from the sale of that specific stock. You could also create a separate portfolio titles 'Sold' and add the former position there. However, that would impact the total for your combined lists.
well unfortunately the complete answer is YOU CAN'T- there are many published "workarounds" one could use to account for stock sales but in reality, Yahoo Finance is simply not offering a full portfolio management solution. all described workarounds require manual operations and tracking that work for the occasional trader. if you have higher needs, look for a better suited tool.