Is it permissible for a wife to use a general power of attorney from her husband for a purpose other than that for which it was intended, and what is the ruling of Islamic law on that?

If a husband appoints his wife as an agent for a specific matter, she is not permitted to exceed the scope of that agency except with his permission. If she acts without his permission, she commits a sin, and her action is Islamically void. She cannot rely on a general agency contract, as it involves gharar (excessive uncertainty/risk).

Ibn Qudamah said: "The agent does not possess the right to act beyond what is necessitated by the principal's permission, whether explicitly stated or by common practice." He also said: "Agency is not valid except for a known transaction. If one says: 'I appoint you as an agent for everything,' or 'for every small and large matter,'... it is not valid. Abu Hanifa and Al-Shafi'i held this view," because it involves great gharar and significant risk. It is stated in Al-Muhadhdhab: "Agency is not permissible except for a known transaction. If one says: 'I appoint you as an agent for every small and large matter,' it is not valid, because it would include what one can and cannot bear, thus greatly increasing gharar and harm."