Anyone who is given something for a specific purpose is not permitted to spend it on another purpose unless with the permission of the one who gave it, and an example of this is
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The questioner : The questioner says: Honorable Shaykh, my husband gives me a personal allowance and an allowance for the house, and I bought a gift for my mother from this allowance. Is this permissible for me?
The Shaykh : Yes, it is permissible for her to do so, on the condition that she seeks her husband's permission. Without his permission, it is not permissible.
The presenter : That is right.
The Shaykh : On this occasion, I would like to say to my brother listeners an important rule: anyone who is given something for a specific purpose is not permitted to spend it on another purpose unless with the permission of the one who gave it. For example, if a person gives you some money and says, "Take this as my contribution to building such-and-such a mosque," it is not permissible for you to spend it on building another mosque unless with his permission. This is because the agent acts within the limits of the principal's permission. He acts in what the principal has permitted him to do, provided that agency is permissible in that matter, and he does not act in what the principal has not permitted unless he obtains his permission.
The Shaykh : Yes, it is permissible for her to do so, on the condition that she seeks her husband's permission. Without his permission, it is not permissible.
The presenter : That is right.
The Shaykh : On this occasion, I would like to say to my brother listeners an important rule: anyone who is given something for a specific purpose is not permitted to spend it on another purpose unless with the permission of the one who gave it. For example, if a person gives you some money and says, "Take this as my contribution to building such-and-such a mosque," it is not permissible for you to spend it on building another mosque unless with his permission. This is because the agent acts within the limits of the principal's permission. He acts in what the principal has permitted him to do, provided that agency is permissible in that matter, and he does not act in what the principal has not permitted unless he obtains his permission.
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، 895- كل من اعطي شيئا لجهة معينة لا يجوز صرفه لجهة اخرى إلا بأذن من اعطاه ومثال ذلك — youtube.com
The Arabic text is a machine transcription of the audio recording.