Śruti has broadly said:
tyen tyaktena bhunjīthā mā gridhah kasyavid dhanam (Isha Upanishad)
Enjoy pleasures with the intention of renouncing them. Share and eat. Do not desire the wealth of others. This has then been specified in Smriti. Śruti is code of conduct. Smriti guides us how to implement the constitution in our life. So Śruti is like the constitution and Smriti is the enforcement of law. There are many agencies to help with enforcement such as police, tax collectors etc. The important thing is that the law must abide to the constitution. So, Smriti is the mode of implementing the constitution/principles in Śruti in our day to day life.
Purāna is the stories of those who have and have not followed their lives according to Śruti. Hence you see the stories of devotees and demons. Lord Ram lived according to Smriti and became the ideal human being. Thus we must live our life like Lord Ram and not like Ravan. All these talks have been mentioned for the well-being of the entire universe.
The topic on taking out ten percent of your wealth for a good cause is in Smriti. We have many Smritis such as Manu Smriti, Parashar Smriti, Yagnavalkya Smriti etc. As times move on, the issues in the society will change, so law and order will need to adapt accordingly. A new Smriti will be developed. Hence there are many Smritis.
Ten percent is lenient. In fact, it is mentioned as twenty percent (one fifth of the earnings).