IHTM33161 | Loss on sale of land: purchases: background
From HM Revenue & Customs · Inheritance Tax Manual
Under IHTA84/S192 you need to adjust the sale price if:
the purchase price (or the aggregate of the purchase prices) does not exceed the aggregate sale prices (IHTM33072) on all qualifying sales.
If the purchase price (or the aggregate of the purchase prices) is more than the sale price, no sale of land relief is due at all.
You should note the following points:
purchases are taken into account only if they are made during the period beginning on the date of death and ending not later than four months after the last qualifying sale
a qualifying sale for this purpose is one that is made within three years of the date of death (no account is taken of any sales in the fourth year IHTA/S197A(3))
the purchase price does not include expenses.