Something like this:
IF 2 > (SELECT COUNT(*) FROM dbo.FACT_TPG_INVENTORY
where STAGING_TABLE_NAME='Staging_SPM_Inventory')
BEGIN
SELECT staging_SPM_Inventory_id, DATE_ID, DIM_DATA_SOURCE_ID, site,
operation_pk, lookup_product_id, quantity, product_group
FROM dbo.Staging_SPM_Inventory
END
ELSE
BEGIN
SELECT staging_SPM_Inventory_id, DATE_ID, DIM_DATA_SOURCE_ID, site,
operation_pk, lookup_product_id, quantity, product_group
FROM dbo.Staging_SPM_Inventory
WHERE dbo.staging_SPM_Inventory.staging_inserted_time >
(SELECT MAX(dbo.fact_tpg_inventory.dw_load_dt)
FROM dbo.fact_tpg_inventory
WHERE STAGING_TABLE_NAME='Staging_SPM_Inventory')
END