Wa `alaykum As-Salamu waRahmatullahi 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.
Answering your question, `Abderrahman Ibn Nasser Al-Barrak, a famous Saudi scholar, states:
It is sufficient for him to spend on them with the intention of paying off what he owes them, and in fact he will have been spending on them from their money, and he does not have to tell them the real situation.
Allah Almighty knows best.