You buy shares in XYZ Ltd for US $50,500.
The instrument transferring the shares to you is executed on 28 October 2018.
Euro-US exchange rate on 28 October 2018: €1 = $1.1
Consideration in US $: $50,500
Euro equivalent of US $ consideration: €45,454.55 (50,500 ÷ 1.1)
You pay Stamp Duty on €45,454.55.