This is what my inventory window code looks like. It's been a while since I've messed with UIs, but perhaps you're referencing a ScreenID that the game doesn't know, since money is native to the inventory window. Many elements are hard coded to specific windows, but I'm not sure if money is among them.
Code:
<Button item="IW_Money0">
<ScreenID>IW_Money0</ScreenID>
<RelativePosition>true</RelativePosition>
<Location>
<X>303</X>
<Y>121</Y>
</Location>
<Size>
<CX>70</CX>
<CY>24</CY>
</Size>
<Style_Transparent>false</Style_Transparent>
<!--<TooltipReference/>-->
<Style_Checkbox>false</Style_Checkbox>
<!--<RadioGroup/>-->
<Text>9999</Text>
<TextColor>
<R>255</R>
<G>255</G>
<B>255</B>
</TextColor>
<Template>BDT_PlatinumCoin</Template>
<DecalOffset>
<X>1</X>
<Y>3</Y>
</DecalOffset>
<DecalSize>
<CX>18</CX>
<CY>18</CY>
</DecalSize>
</Button>