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HMRC internal manual

Stamp Duty Land Tax Manual

Scope: How much is chargeable: Contingent, uncertain or unascertained consideration: Adjustments under FA03/S80: General

The amount of stamp duty land tax payable can be adjusted when the amount of the consideration is determined in cases where the tax was paid on the basis that the whole or part of the consideration for the transaction was contingent, uncertain, or unascertained at the outset.

See the example at SDLTM05040a.