What are the sources for taking 10% of your salary and giving it to tzedakah? Is this a must or something that is encouraged and is there a source for those that do not?

The origin of tithing ten percent from ones income – Ma’aser Ksafim – is disputable, some holds it’s a Torah commandment, some hold it’s a rabbinic commandment and some, it is a practiced custom. We hold like the last opinion. One should remember that the mitzvah of Tzedakah is clearly from Torah it’s only the percentage that is a custom. Therefore, those who for some reason can’t give as much as ten percent can give less. (See Tzitz Eliezer 9, 1)