I have received an offer from a company (startup) [Location - India]. They are paying me well but the discussion we had was over a call so I have not received the offer letter.

I have a over a month of notice period in my current company. I am not interested to join them but would like to leverage the offer while I'm interviewing with other companies. To negotiate with other companies I would probably need to show the competing offer letter.

I think this company will only send me the offer letter once I give a verbal confirmation to join them.

My dillemma is more moral / ethical. Should I give a verbal confirmation only to tell them that I won't be joining as soon as I get the letter? How is this perceived? And does it matter (networking, reputation etc)?