Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
2- To expiate for this, Islam stated three consequent options; the first of which is to free a slave, which is not available nowadays. The second is to fast sixty continuous days. If the husband cannot do these two, he must resort to the last option which is feeding sixty poor people.
Responding to your question, Dr. Main Al-Qudah, Assistant Professor, AOU, IAU, SAA, Member of AMJA Fatwa Committee, and Imam, MAS Katy Center, states:
The first option of expiation for having full sexual intercourse between spouses during obligatory fasting hours is to free a slave (when there was slavery).
If the husband cannot do that due to any legitimate reason (like lacking slaves as it is the situation nowadays), then he has to fast sixty continuous days.
If the husband cannot do that either, he has to feed sixty poor people.
As for the wife, she has to make up for that day according to an opinion attributed to Imam al-Shafi`i.
The above expiation is required in addition to making up for the day they missed.
Almighty Allah knows best.