Musar for Ketubot 99:17

A man should be generous with his money to copy and to purchase books and to lend them to one who has none, as it is said, "Wealth and riches are in his house; and his merit endureth for ever" (Ps. 112:3). And a man should be easygoing and not mind if the books are damaged because of their use (Kethuboth 50a). He should also be generous when it comes to his household utensils, and lend them to his neighbors and his friends.

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.