What is meant by the "expenditure" mentioned in the Second Pledge of Al-Aqabah, and is it the expenditure on the nascent Prophetic State, which is replaced today by the security and development tax?

Taxes imposed for the public good, which do not involve injustice or aggression, are legitimate and must be complied with. As for the spending mentioned in the Second Pledge of Aqabah, it refers to spending in jihad, which is consistent with the interpretation of the Almighty's saying: "And spend in the way of Allah."