Is it possible that he meant you need 2 checks from the bank? One for $10k that would be out of the mortgage itself? And the rest in a separate account but all added up to the same amount you agreed to?
If not, your lawyer has not done his job right. I would even bet on it being unethical if he expects you to pay extra after you have explained what you would and wound not do. It could be a reportable (to the bar) offense if it is.
Good luck to you - buying a home can be very stressful but the ownership of your own home is worth the stress!