Offer from Deloitte

This is my first switch and first offer cleared written technical round 1 n techno managerial round n HR round. After getting selected they are not matching my expectation and asking me to put resignation for DoJ which is in Dec mid week. I already discussed that my notice period is of 90 days and given Doj is of of 55th day before the 90 days np but they told me to negotiate your Hr first and let them know they will change DoJ as per the last working day given on releasing letter.

I find risk in this scenario if TCS doesn’t retain me and Hr might refuse later to extend the DoJ?

I desperately want to be out of TCS, what should I do? Ps: 1 offer already released with which needs a location change