Is it permissible to feed ten poor people instead of fulfilling a vow to pray one hundred rak'ahs due to inability to perform them?

If you are unable to fulfill the vow, then you must perform an expiation for an oath (kaffarat yamin): freeing a slave, or feeding ten poor persons from the average of what you feed your families, or clothing them. If you are unable to do that, then fast for three days. The Prophet, peace and blessings be upon him, said: "Whoever makes a vow that he can fulfill, let him fulfill it."