Safest return option

So I'm about to receive around 7 lakhs as retention bonus from my current employer. But I've already decided to quit the company and I also have another offer that pays 60% more than what I'll be making here. I'll get the amount in my bank account by 30th December. I'll be resigning by 3rd January, post that I'm liable to return the entire sum by 25th Jan.

What is the safest way I can make as much returns as I can on this money in that timeframe?

Thanks in advance!