freight reimbursement applicability to service tax

A seller (sole proprietory concern) supplying to multinational entity a Ltd concern.Freight is paid in advance to transporter and added in the invoice itself by the seller..Buyer ( Ltd concern) later pays total invoice including freight to the seller (sole proprietory concern) and mentions cost and freight part seperately in the purchase order

now who is liable to pay service tax in this case as freight is later reimbursed by buyer to the seller

regards