Dear,
The retainer will raise the invoice to your company/firm, which will include service tax (the retainer will have to get themselves registered for service tax under the appropriate head, e.g., professional consultant). Once the invoice is raised by the retainer, the company/firm would make the payment as per agreed terms, subject to deduction of TDS.
A typical agreement could be as follows:
Dear [Recipient's Name],
We are pleased to appoint you as a professional consultant/advisor on a retainer basis from dd/mm/yyyy.
The following are the terms and conditions:
a) The retainer shall be from dd/mm/yy to dd/mm/yy. It will automatically expire on dd/mm/yy unless extended in writing. During the retainer period, either side can terminate the agreement by giving ... months' notice or retainer charges for the notice period, without assigning any reasons.
b) You shall interact with Mrs./Miss/Mr. [Contact Person's Name] of our company during your retainer for all decisions/queries.
c) You shall be given... number of persons for your assignment.
d) Your assignment shall be as follows...
e) Your retainer fees would be Rs... per month. You would raise an invoice on the ...th of every month. Your invoice will become payable by... of every month. You may levy service tax/your retainer fees are inclusive of service tax.
f) You shall be paid conveyance for official purposes.
g) Any outstation travel and boarding shall be billed extra to the company/firm. Limits on traveling and boarding expenses are as follows...
h) Other clauses.
Kindly sign and return a duplicate copy of this letter as a token of acceptance.
Thanking you,
Yours sincerely
Hope the above gives you a fair idea.
Regards