What is the ruling on lying to someone who wants to buy goods from me, when this person procrastinates in repaying debts, and I do not want to sell to him for fear of his procrastination, knowing that informing him of the unavailability of goods is a lie?

Lying is forbidden (haram (definition)) and is a characteristic of hypocrites. There is no such thing as "white" or "black" lies; all lying is reprehensible. Lying is permissible in certain specific situations, but the present case is not one of them. It is not permissible for you to lie to this person. Rather, be frank with him and tell him that the reason is the procrastination you fear from him, doing so in a wise manner that combines advice, honesty, and safeguarding the merchandise. You may use tawriya (ambiguity/double meaning), such as saying: "Indeed, you do not sell on credit," intending that you do not sell on credit to him. For it has been authentically narrated from Imran ibn Husayn: "Indeed, in ma'arid (ambiguous statements) there is a latitude (or escape) from lying."