CAn you update your products by SKU instead of product/variant ID?
<AspDotNetStorefrontImport Verbose="false">
<Transaction>
<Product Action="Update" SKU="1">
<Variants>
<Variant Action="Update" SKU="143">
<Inventory>100</Inventory>
</Variant>
</Variants>
</Product>
</Transaction>
</AspDotNetStorefrontImport>