Right or Wrong!

I was on a PIP. I was given 3 months with 3 goals to achieve in each month. I found an opportunity outside the organisation so I sent my resignation to the manager. During the discussion she promised me I would be paid 2 months basic salary! But on the full and final settlement day neither finance nor HR were aware of this. So they told me this promise without which was made orally does not have any value and it was against the policy! I lost almost 1.8L. The manager did not have the guts to reply to the same email where I asked for the clarification instead she replied to me on the WhatsApp saying she meant two months salary which I was supposed to be paid to them for not serving the NP would be waived off! I know the basic difference between "waived off" and " will be given/paid". I trusted her words blindly.