It is becoming increasingly common for real estate agents to include a special conditions requiring the buyer to pay the balance commission at settlement when the deposit doesn't cover the agent's full commission. However, unless the terms satisfy specific and narrow elements, they are not worth the paper they're written on, they're completely unenforceable. 

More and more buyers and sellers are signing their property contract electronically, with DocuSign being the most common tool being used. If not done properly there can be serious consequences including the contract being void.