Got an Offer Letter but Don't Want to Resign Yet – Need Advice on Managing HR Tactfully

Hey folks,

I recently received an offer letter from a company, but I'm hesitant to resign from my current job just yet. The main reason is that I'm expecting a potentially better offer next week and want to keep this one as a backup.

The challenge is that the HR from the company that gave me the offer is now following up on my Date of Joining (DOJ) and might soon ask for resignation proof from my current employer.

Has anyone been in a similar situation?

Can anyone suggest me how to tackle this situation?