Answer
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:
It is permissible for your son-in-law to pay your share in the udhiyah as a reward for your husband’s effort in moderating his work, for which your husband refuses to take any salary.
Responding to your question, Dr. `Abdul-Fattah Idrees, Professor of Usul Al-Fiqh, Al-Azhar University, stated,
Slaughtering a sacrificial animal, udhiyah, is the most recommended deed a Muslim can do on the day of `Eid Al-Adha, for that is part of Islam’s rituals and a mode of seeking Allah’s Pleasure, as Allah said, “…So pray unto they Lord, and sacrifice…” (Al-Kawthar 108:2)
So, all Muslims are highly encouraged to make udhiyah if they can in order to get the great reward from Allah the Exalted and to share in feeding the poor on the days of `Eid.
It is permissible for your son-in-law to pay your share in the udhiyah as a reward for your husband’s effort in moderating his work, for which your husband refuses to take any salary.
As for the money, you still have as a trust (amanah), you can pay it in any charity either run by your son-in-law or any other person, as long as it is a trustworthy charity.
Almighty Allah knows best.