Shall PO be approved by Finance also? before sending to supplier ?

No , PO is only Approved by the authorized persons in each entity according to the chart of any entity , but most of entities and the common is by Purchasing Or Supply chain Or Operations team Department as finance not an approval before supplier but its any overview after its already made and released to supplier , but its apply the concept of3 match , by invoices and Goods / services deliver and the PO.