I’ve been through so many shenanigans during my previous life as a naive startup employee: paid huge amounts of AMT (which took years to recoup via AMT credits), was not offered 83b election, had to write huge checks to exercise ISO, had to pay taxes when exercising NSO, etc., but I had never heard of a company threatening to forfeit the RSU if tax is not wired to them, it’s simply wild, especially when the liquidity is so close.
If I was in your shoes and the amount was substantial, I’d consult a lawyer. I hired one to help me facilitate a secondary sale transaction contract and it cost me $4k, money well spent. Tons of them in the Bay Area.
I would truly love to know how this will end up for you.