My paternal aunt has given us many gifts (like air conditioner) using her husband's money, while he was not aware. Her husband gave her the money to use for her household things but she gave the extra money to us. Is this regarded as stone money? Do we have to return all the gifts?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is not permissible for your aunt to take anything from her husband's money without his knowledge to give it to you, based on Abu Dawood and Al-Nasaa'i's report on the authority of 'Abdullah ibn 'Amr that the the Prophet said: “It is not permissible for a woman to give anything (of her husband's property) except with the husband's permission.” At-Tirmidhi reported the hadeeth of Abi Umamah Al-Bahili in the Farewell sermon, “A woman must not give anything away from her husband's house without her husband's permission.”

And what your aunt's husband gives her of money to spend on household items, then she is entrusted in it, so it is not permissible for her to utilize it except in what the husband specified, except in charity (Sadaqah) in small amount for what is generally permitted or it is customary to tolerate it, based on the Prophet's  saying: “When a woman gives some of the food in her house, without wasting it (i.e. being extravagant), she will have her reward for what she has given, and her husband will have his for what he earned. The same applies to a storekeeper. In no respect does the one diminish the reward of the other.” [Muslim] Al-Nawawi said in Sharh Muslim: “And know that all of this is supposed to be in a small amount, for which usually the owner's satisfaction is known, but if the amount exceeds what is customary, then it is not permissible. And this is the meaning of his saying : “If a woman gives in charity from her households without wasting (i.e. being extravagant)…”

Based on your saying: “My aunt gave us many gifts (such as an air conditioner)”, it appears to us that the money given to you is a lot, and that is not permissible except with a specific permission from the husband.

You are obligated to do one of two things: Either you tell your aunt's husband about these gifts and if he is consent with that, then that is what is required of you, or you return the gifts to her, except for the small things for which it is known that your aunt's husband would be pleased with.

Allah knows best.