I'm selling my house to my spouse. He is using some of the equity as his downpayment. It's a gift from me to him. Is there a penalty/tax for either of us at some point?

There are no tax consequences for gifts between spouses, but you should consult with local legal counsel.

See https://www.avvo.com/divorce-separation-lawyer.html

"I'm selling my house to my spouse"

You should consult a local tax professional.  If you sell the home below its fair market value ("FMV"), you're giving the buyer a "gift of equity", which may require you to file a gift tax return.  Also, if you realize a capital gain from the transaction, you may be liable for a capital gains tax.

https://www.investopedia.com/terms/g/gift_of_equity.asp#:~:text=A%20gift%20of%20equity%20is,the%20actual%20gift%20of%20equity.

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@TomD8 wrote:

If you sell the home below its fair market value ("FMV"), you're giving the buyer a "gift of equity", which may require you to file a gift tax return. 

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A gift tax return (Form 709) for a gift to a spouse is required only in very limited circumstances, which most likely would not be applicable here.

See https://www.irs.gov/instructions/i709#en_US_2022_publink16784xd0e489