Cst waybill for free goods

Hi,

I am working in a Pvt Ltd company and our company is a Dealer for a Heavy Machinery Equipments in Hyderabad, Telangana State and our Principal Manufacturers cum Suppliers are at Pune, Maharastra State. Our Prinicipal Suppliers sends few parts regularly under Free of Cost (FOC) for the equipment warranty purpose and for that, they issues their invoice to us as "Warranty Invoice" along with material value and against that invoice, they collects our advanced CST waybill and dispatch their goods. the same goods we collects from the transport and issues to our customers (who owns the machinery) with out our sale invoice.  As per my knowledger if we issue any advanced CST e-way bill, the sales tax department may treat as purchase and accordingly we have to show our sale invoice. But, in this case we are not issueing any sale invoice. Can we issue our advanced E-waybill for that FOC invoice? 

Please clarify me in this regard.