Can a seller refuse to release escrowed funds even after the holdback period technically ends?

Generally no, if the escrow is properly structured, since release is not supposed to be within either party's unilateral control in the first place. In a typical escrow arrangement, the funds are held by a neutral third party, often a law firm or trust company, under an escrow agreement that specifies exactly when and how release is supposed to happen, commonly release at the stated deadline unless a valid notice of claim was delivered before that date under the agreement's own procedure.

Because the seller is not the one holding the funds, the seller alone generally has no direct power to simply refuse or block release once the agreement's conditions for release have actually been met. What can genuinely hold up release is a valid, timely notice of a claim from the buyer, which most escrow agreements treat as sufficient to keep the disputed portion held pending resolution, but that is different from a seller unilaterally objecting without a properly asserted claim. A seller facing an unexplained delay in release should check whether a valid notice was actually delivered, and if not, follow up with the escrow agent directly.