Just a guess (I'm not an attorney), but in some jurisdictions, emails showing the agreed-upon price and the later change will show bad faith on their part. Contracts don't have to be in writing, if you can show evidence that there was "a meeting of the minds". IOW, if it's apparent on the earlier email that you were both in full agreement about the price, most judges will consider that sufficient. (Give it a few years, and we'll be able to legally sign things online - then legally signed emails will hold up in court. Way back in March 2013, I signed the contract to buy my house online. And I run signature software on my phone. But some jurisdictions haven't caught up with technology yet.)