If your 5% at the new valuation is worth a lot more than $100K, I would say don't do it unless you need the money in the near term.

For example, if XX is 10, 5% of $10 million is $500K.

If you do need the money, maybe you should make a counter offer that is a better deal for you.

And don't forget you have to pay income tax this year if you do sell. If you wait, obviously you also delay the tax.