IHTM14874 | Lifetime transfers: future payments: payments outstanding at death
From HM Revenue & Customs · Inheritance Tax Manual
Where the transferor dies before all the payments have been made, it falls upon the legal personal representatives (IHTM05012) to complete those payments.
You can
give an allowance in valuing the transferor’s estate immediately before the death for the non-gratuitous part of the remaining payments, but
charge tax on the gratuitous portion.
Review the separate instructions at the liabilities section of the manual. (IHTM28110)