For a sale I would expect you would not only require unanimous vote but unanimous agreement from those who did not vote as well. The common property belongs to everyone and so every owner needs to agree to the sale.
If you have already spent $20,000 and the sale has not completed yet your costs are going to be higher than this. Also if you have spent $20,000 on advise why haven’t they told you what sort of approval is needed.
Seems to me you should cut your losses and abandon the sale because the buyer is never going to pay $15,000 plus costs $20,000 +.
The buyer should have agreed to pay these costs before any spending was done not the EC.