RTS Resource handling and transactions?

So every building and character has an instance of this class:

 public class nInventory : MonoBehaviour
     {
         public int food = 0;
         public int logs = 0;
         public int lumber = 0;
         public int stone = 0;
         public int bricks = 0;
     }

In my game an NPC must be present at a certain building in order to retrieve items from it.

So when they get hungry, they go home and get food. If they need wood they go to a lumber yard, etc etc etc

My predicament is this:

Lets say NPC1 needs 30 wood, and NPC2 needs 50 wood.

Lumberyard1 has 60 wood.

Now whoever gets their first is going to end up getting their wood, leaving the other guy to reach the destination realizing that he does not have enough wood to accomplish his task.

I'm running into an issue where this happens A LOT. NPC's running around thinking they can retrieve something only to get to their destination and find nothing.

My question is this:

What is an efficient way for NPC1 to "call dibs" on his resources before he actually gets to the destination?

Effectively removing what he needs from the available amount, so that when NPC2 checks where resources are at, they will not see the resources that NPC1 is on his way to pick up.

This turned out longer than I had intended.