Musar על כתובות 99:2

Perhaps the answer is similar to the statement by our sages in Ketuvot 67 that if someone wishes to be generous in his charitable gifts, He should not give away more than 20% of his wealth so as not to risk becoming poor and dependent on others. Wealthy people are able to donate 20% of their wealth. What they are left with will still be ample. When one has limited means, however, giving away 20% of those means may represent a great hardship. When Rashi speaks about something being beyond a person's ability he refers to something beyond a person's ability to part with without suffering hardship as a result.