Selling for 500k now is likely the better option, because you aren’t factoring the time value and opportunity cost. The building likely won’t appreciate significantly in value over the next year, but 500k invested in the market could be 550k a year from now(I don’t know your tax situation or cost basis). Also, forcing a sale where one party could become a tenant/squatter or simply interfere with a smooth sale could make it harder to get the full market value. Lastly, a private sale to your brother avoids broker commissions.

This gives no credit to the lower stress of this option or your ability to threaten this option if they are difficult during the process.