How can I pay zakat on money I lent to someone, knowing that the time for its recovery has not yet come and I do not have sufficient funds to pay zakat on it?

This money you lent to your friend is subject to Zakat. You can delay paying the Zakat until you receive the money, then pay its Zakat for the past years. This applies if your friend is financially well-off and honorable. However, if he is in financial difficulty, or denies his debt, or procrastinates, then there is a difference of opinion regarding the obligation of Zakat on this money. Some say you should pay its Zakat for all past years upon receiving it, which is more cautious. Others say you should pay its Zakat for one year upon receiving it (this is the view of the Malikis). And still others say Zakat is not obligatory until one year has passed from the day you receive it (this is the view of the Hanafis).