The ruling on one who affirmed his promise with an oath to give something and did not do so
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The listener Ali Muhammad Ali, a Sudanese resident of al-Majma'ah, sent a message saying: I lost some valuable items, so I swore by the Great Allah that whoever finds them I will give a reward. A man found them, but I have not given him anything up to now. Is there a sin on me for this? And is it obligatory for me to give him the reward? Please inform us, may Allah reward you with good.
Answer: It is prescribed for you to fulfill your promise and what you swore to, and to give him the appropriate reward. If you do not give it to him, you must pay the expiation for the oath because you swore. Either you give him the reward, or you pay the expiation for the oath. It is prescribed for you to give him the reward because you promised and swore to do so, so you should fulfill it. The believer, when he promises, fulfills it. Yes.
The presenter: May Allah reward you with good.
Answer: It is prescribed for you to fulfill your promise and what you swore to, and to give him the appropriate reward. If you do not give it to him, you must pay the expiation for the oath because you swore. Either you give him the reward, or you pay the expiation for the oath. It is prescribed for you to give him the reward because you promised and swore to do so, so you should fulfill it. The believer, when he promises, fulfills it. Yes.
The presenter: May Allah reward you with good.
Source
binbaz.org.sa
The Arabic text is copied verbatim from the original source, without any edits.