What happens to a pending sale if a beneficiary contests the will partway through the deal?

It depends heavily on the stage the sale and the estate administration are at. If the trustee already has a valid Certificate of Appointment and the challenge comes after that authority is established, a pending sale can often still proceed, since a will challenge alone doesn't automatically strip the trustee of authority already granted, though a court could intervene in serious cases. If the challenge happens before a certificate has been issued, it can be far more disruptive, since a formal objection filed with the court can block a certificate of appointment being issued on the will in question, which in turn stalls any closing that depends on that authority being confirmed.

A sale already under contract can also face practical problems even if it isn't legally blocked outright, since a buyer's lawyer who becomes aware of a live will dispute will often want reassurance about the trustee's authority before closing. Because outcomes vary so much based on timing, the specific ground for the challenge, and whether a certificate has already issued, an estate trustee facing this situation should get legal advice immediately to understand whether the sale can proceed, needs to be paused, or requires court direction.