So, you sold your property direct to a former partner, without a Realtor, and how you are having second thoughts about the price?
Sorry, but IMHO unless there was fraud involved if you agreed on a price, it's over.
This is not legal advice. Get an attorney, then if you feel you are wrong, sue for the difference. But, in hindsight, wouldn't you feel better if you had the transaction properly handled? I see similar situations. It is always AFTER the fact...before it's just "Let's save money on the commission".