Invoice or debit memo

Dear All,

Happy "New Year" to Every One.......

Here I have one doubt about the accounting treatment.

We need to get the reimbursement from our client, for that we need to submit an invoice to the client or raise the debit memo which one is correct this is the question for u and particulars r given below.

We are getting a business for that we travelled to our client (different branches) places and given a demo about our service now finally the client accepted the business from us and them given an opportunity for reimbursement of travel expenses also for the visit of branches of client.

Please keep in mind we are a small service provider and we are not registered under the Service Tax because of startup company and now please advise me which is best one either invoice or debit memo and we need to think about service tax point also (either affect the service tax or not).

Thanks in advance for your valuable answers and suggestion.