The Torah affirms the importance of charity:
עשר תעשר – you shall tithe… (14:22)
A double statement means to repeatedly do it, an unlimited amount of times. The difficulty this poses is that the Gemara in Kesubos caps the permissible amount of charity at no more than 20% income. These are mutually exclusive concepts.
The Vilna Gaon deduces that if the Torah requires endless generosity, it can only be that the reward for charity is the ability to give more, without hindering the giver. The Gemara in Taanis therefore says that עשר בשביל שתתעשר – a person will never be limited in their ability to to give charity over time.