I don’t think this is specific to Sharia law, those rules on lending is for Islam in general. I think giving away 30% of your income to the poor every year is one of the five pillars of Islam as well.
There is no set amount or percentage dictated by the Quran. Usually the customary amount is ~2.5% of capital assets earned in a year. And apparently somewhere around 20% on mining income, which obviously is only applicable to small number of people who own interests in mining operations.
1.) Islamic law is the same as Sharia law
2.) It's 2%, and in the past often collected as tax.
It's 2.5%
Yeah, my bad. But still nowhere near the 30% mentioned by the original comment.