On Monday morning, a buyer agent who works for ABC Realty submits a buyer client's...

On Monday morning, a buyer agent who works for ABC Realty submits a buyer client's offer to the listing agent at XYZ Realty. Later that day, the listing agent presents the offer to the seller. On Tuesday morning, the seller signs the offer with no changes, and on Tuesday evening, the listing agent calls the buyer agent to tell them the offer is signed and accepted. The buyer agent finally gets in contact with the buyer Wednesday morning to communicate the good news. On Wednesday afternoon, the listing agent delivers the signed offer to the buyer agent, and the buyer agent delivers it to the buyer that evening. At what point was there a valid, binding contract between buyer and seller?