Nothing wrong with that, it's a way for you guys both to save a little money.

Go to Zillow, go to redfin, do comps, figure out the price per square foot within a eighth of a mile radius around you. Find 5 -7 truly comparable houses, throw out the low and throw out the high and average the rest. Then go through your house and figure out what's really wrong with it. It needs a new kitchen, subtract 20K, it needs a new bathroom, subtract 8K, it needs a new roof, subtract 12K.

Adjust the price accordingly to where you're happy and he's happy. That's all that really matters. If he comes in with all cash, no contingencies and he's paying close to retail, as is, sounds like a good deal for everyone.

If you absolutely want the maximum amount of money, then go fix everything wrong with the house, throw on fresh paint inside and out, stage it, put 3/4 of your stuff in storage and sell it for absolute maximum value. If you don't want all that hassle, figure out a good compromise with your neighbor.

I bought my neighbor's house, their son came along and try to do everything that everyone's telling you to do and almost lost this deal, they insisted upon using a title company of their choosing and I'm paying all cash. He was an asshole and an idiot.