IHTM23205 | Special valuation matters: purchaser's interest under a binding contract to purchase
From HM Revenue & Customs · Inheritance Tax Manual
Where a purchaser of a property dies between
entering into an enforceable contract to purchase, and
the completion of the conveyance
the interest of his estate is in the property itself not the cash used to purchase it.
You should therefore refer the property to the VOA (IHTM23002) in the normal way.
In Scotland if the purchaser has not obtained a registered title to the property, the matter should be referred to TG for advice before any reference to the VOA is made if the point is likely to be material.