I think the author may consider restraint and have patience in awaiting further communication from the concerned manager without advertising the matter too much in the public domain. Such an action is likely to be not liked much by the company. Also, the things can be taken as finalized only after joining the company on receipt of a firm offer .
Since the author has not yet been finally selected by the company , no rules and regulations apply at this stage. The private companies are free to take their own decision regarding selection of an employee for their organization on payment basis. No external agency can intervene in such matters.
Regarding posting a response to the mail/letter received, the same should be acknowledged politely and a willingness to join the company on receipt of a firm offer should be reiterated.