What do you tithe from? Why tithe income? According to Scripture, we should pay tithe from increase from our land and cattle. I don't find anything about tithing monetary income, so why do you pay from income or salary? 

Also, if the income you receive has already been tithed, should you tithe again?

For example, someone tithes $100 on $1,000 income and is left with $900. From $900, that person pays $500 to another person for some work. Should the other person pay tithe for receiving $500?
Tithing income is an application of the Old Testament tithe principle - it's giving back 10% of what God has given you. If that is cows, great! If it is money (as it is in modern society), then tithe comes out of that.

The 10% number is not actually mentioned in the New Testament, where the principle is just "give generously!"

Based on the above, I tithe on all my "income". If I receive $500 from someone else for work done, yes I would tithe on it. On the other hand, I don't tithe my tax refund

One other thing - I believe that tithing should be done on gross (before-tax) and not net income. This is an application of the "firstfruits" principle from the OT - our tithe is to be "off the top". Tithing on your net income is letting the government have the first cut and tithing on what is left over.