Resigned TCS ,got hik3 offer

Hi ,everyone I'm have recently instlled this app,I'm working as java backend developer. Digital candidate, I got offer from another company . Now my delivery manager from onsite calls and says they will give 30% immediate hike and onsite opportunity next year . What's ur thoughts on this guys

20mo ago

SwirlyPanda

Cognizant

20mo

Accept only after you receive revised compensation letter and onsite opportunity in written form

SillyNarwhal

KPMG

20mo

I was thinking even though if I get in written form ,they might change anytime.
And another thing till now they didn't say suddenly I'm hearing this after my resignation . How legit it is