What must you do before having sex with a woman that you remarried after previously divorcing (by saying she was like your mother)?

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Set a slave free

Those who put away their wives (by saying they are as their mothers) and afterward would go back on that which they have said, (the penalty) in that case (is) the freeing of a slave before they touch one another. Quran 58:3

If you can't afford a slave, fast for two months.

And he who findeth not (the wherewithal), let him fast for two successive months before they touch one another. Quran 58:3

If you're unable to fast, feed sixty poor people.

And for him who is unable to do so (the penance is) the feeding of sixty needy ones. Quran 58:4b

Then you can go ahead and have sex with her.