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".
I'm not a lawyer, but at this point if you believe that you were presented with falsified information that led you to sell for less than you should have, probably your only recourse is a lawyer.
However, who paid for the appraisal? If you paid for the appraisal, then you deserve to have a copy of the appraisal. Contact the appraiser and ask for one.
And if you didn't pay for the appraisal, why not? Appraisals can be, um, nudged up or down a bit depending on the purpose of the appraisal and who's paying for them. Not necessarily a whole lot, and appraisers are, by and large, very ethical. But there generally is a bit of wiggle room. If the appraiser knew that his/her client was looking for a low appraisal, it wouldn't be suprising if the appraisal came back somewhat lower than if the appraiser knew the client was looking for a high appraisal.
So: If you paid for it, ask the appraiser. Check the documents you have; do you have a copy of the actual appraisal? If those suggestions don't work, contact a lawyer.
Hope that helps.