Paper/paper-server/nms-patches/net/minecraft/world/item/trading/MerchantRecipe.patch

--- a/net/minecraft/world/item/trading/MerchantRecipe.java
+++ b/net/minecraft/world/item/trading/MerchantRecipe.java
@@ -8,6 +8,8 @@
 import net.minecraft.util.MathHelper;
 import net.minecraft.world.item.ItemStack;
 
+import org.bukkit.craftbukkit.inventory.CraftMerchantRecipe; // CraftBukkit
+
 public class MerchantRecipe {
 
     public static final Codec<MerchantRecipe> CODEC = RecordCodecBuilder.create((instance) -> {
@@ -44,6 +46,18 @@
     public int demand;
     public float priceMultiplier;
     public int xp;
+    // CraftBukkit start
+    private CraftMerchantRecipe bukkitHandle;
+
+    public CraftMerchantRecipe asBukkit() {
+        return (bukkitHandle == null) ? bukkitHandle = new CraftMerchantRecipe(this) : bukkitHandle;
+    }
+
+    public MerchantRecipe(ItemCost baseCostA, Optional<ItemCost> costB, ItemStack result, int uses, int maxUses, int experience, float priceMultiplier, int demand, CraftMerchantRecipe bukkit) {
+        this(baseCostA, costB, result, uses, maxUses, experience, priceMultiplier, demand);
+        this.bukkitHandle = bukkit;
+    }
+    // CraftBukkit end
 
     private MerchantRecipe(ItemCost itemcost, Optional<ItemCost> optional, ItemStack itemstack, int i, int j, boolean flag, int k, int l, float f, int i1) {
         this.baseCostA = itemcost;
@@ -185,7 +199,11 @@
         if (!this.satisfiedBy(itemstack, itemstack1)) {
             return false;
         } else {
-            itemstack.shrink(this.getCostA().getCount());
+            // CraftBukkit start
+            if (!this.getCostA().isEmpty()) {
+                itemstack.shrink(this.getCostA().getCount());
+            }
+            // CraftBukkit end
             if (!this.getCostB().isEmpty()) {
                 itemstack1.shrink(this.getCostB().getCount());
             }