A signed contract is binding, and a seller generally cannot rewrite the terms just because they now want more time. In most states, and California in particular, a seller cannot walk away from or renegotiate a binding contract simply because you will not agree to a new demand like an extended rent-back after closing. Practically, your first line is your agent conveying the consequences to the seller's agent, with the brokers stepping in if it escalates. The hard part is deciding how far to push if the seller simply refuses to move out. Many contracts require mediation or arbitration before court, and that path can be slow and painful even when you are clearly in the right. Winning the legal fight does not always mean winning the practical one; grinding a difficult holdover seller through lawyers and hearings can cost more time and stress than a negotiated resolution would. Weigh that trade-off honestly, and get advice from a real estate attorney on your specific contract before you dig in. This is a legal question, and the exact language in your agreement controls the outcome.