diff --git a/public/app.js b/public/app.js
index 256d506..75338f4 100644
--- a/public/app.js
+++ b/public/app.js
@@ -121,7 +121,7 @@ class OrderCard {
         idCell.textContent = item.product_id ?? item.id;
 
         const productCell = document.createElement('td');
-        const productSelect = this.buildProductSelect(item);
+        const productSelect = this.buildProductSelect(item.product_id);
         productSelect.classList.add('form-select-sm');
         productSelect.dataset.action = 'item-product';
         productSelect.dataset.itemId = item.id;
@@ -220,7 +220,12 @@ class OrderCard {
         const productLabel = document.createElement('label');
         productLabel.className = 'form-label';
         productLabel.textContent = 'Товар';
-        const productSelect = this.buildProductSelect();
+        const usedProductIds = new Set(
+            (this.order.items || [])
+                .map((item) => item.product_id)
+                .filter((value) => value !== null && value !== undefined)
+        );
+        const productSelect = this.buildProductSelect(undefined, usedProductIds);
         const productSelectId = `add-item-product-${this.order.id}`;
         productSelect.name = 'productId';
         productSelect.id = productSelectId;
@@ -253,6 +258,16 @@ class OrderCard {
         addButton.textContent = 'Добавить позицию';
         actionCol.appendChild(addButton);
 
+        if (productSelect.options.length === 0) {
+            const placeholder = document.createElement('option');
+            placeholder.value = '';
+            placeholder.textContent = 'Нет доступных позиций';
+            productSelect.appendChild(placeholder);
+            productSelect.disabled = true;
+            quantityInput.disabled = true;
+            addButton.disabled = true;
+        }
+
         form.appendChild(productCol);
         form.appendChild(quantityCol);
         form.appendChild(actionCol);
@@ -260,10 +275,13 @@ class OrderCard {
         return form;
     }
 
-    buildProductSelect(selectedId) {
+    buildProductSelect(selectedId, excludedIds) {
         const select = document.createElement('select');
         select.className = 'form-select';
         this.products.forEach((product) => {
+            if (excludedIds && excludedIds.has(product.id)) {
+                return;
+            }
             const option = document.createElement('option');
             option.value = product.id;
             option.textContent = product.name;
@@ -313,7 +331,7 @@ async function fetchJson(url, options) {
         const payload = await response.json().catch(() => ({
             message: 'Ошибка запроса.'
         }));
-        throw new Error(payload.message || 'Ошибка запроса.');
+        throw new Error(payload.error || payload.message || 'Error');
     }
     if (response.status === 204) {
         return null;
@@ -549,18 +567,24 @@ function initializeApp() {
                         `[data-action="item-product"][data-item-id="${itemId}"]`
                     );
 
-                    await fetchJson(`/api/orders/${orderId}/items/${itemId}`, {
-                        method: 'PUT',
-                        headers: {
-                            'Content-Type': 'application/json'
-                        },
-                        body: JSON.stringify({
-                            productId: productSelect.value,
-                            quantity: Number.parseInt(quantityInput.value, 10)
-                        })
-                    });
-                    await refreshData();
-                    return;
+                    try {
+                        await fetchJson(`/api/orders/${orderId}/items/${itemId}`, {
+                            method: 'PUT',
+                            headers: {
+                                'Content-Type': 'application/json'
+                            },
+                            body: JSON.stringify({
+                                productId: productSelect.value,
+                                quantity: Number.parseInt(quantityInput.value, 10)
+                            })
+                        });
+                        await refreshData();
+                        return;
+                    } catch (error) {
+                        showNotification(error.message, 'warning');
+                        await refreshData();
+                        return;
+                    }
                 }
                 case 'delete-item': {
                     await fetchJson(`/api/orders/${orderId}/items/${itemId}`, {
diff --git a/server/db/database.js b/server/db/database.js
index 5d11592..066b0cb 100644
--- a/server/db/database.js
+++ b/server/db/database.js
@@ -384,10 +384,17 @@ export default class DBAdapter {
         }
     }
 
-    async updateOrderItem({ itemId, quantity }) {
+    async updateOrderItem({ itemId, quantity, productId }) {
         let connection;
         
         try {
+            if (!itemId || !Number.isFinite(quantity) || quantity <= 0) {
+                return Promise.reject({
+                    type: DB_USER_ERROR,
+                    error: new Error("Invalid item or quantity")
+                });
+            }
+
             connection = await this.#pool.getConnection();
             await connection.beginTransaction();
             
@@ -406,6 +413,51 @@ export default class DBAdapter {
                 });
             }
 
+            const targetProductId = productId ?? itemRow.product_id;
+
+            if (targetProductId !== itemRow.product_id) {
+                const newProduct = await connection.query(
+                    'SELECT quantity FROM products WHERE id = ? FOR UPDATE',
+                    [targetProductId]
+                );
+                const newProductRow = newProduct?.[0];
+                if (!newProductRow) {
+                    await connection.rollback();
+                    connection.release();
+                    return Promise.reject({
+                        type: DB_USER_ERROR,
+                        error: new Error("Product not found")
+                    });
+                }
+                if (newProductRow.quantity < quantity) {
+                    await connection.rollback();
+                    connection.release();
+                    return Promise.reject({
+                        type: DB_USER_ERROR,
+                        error: new Error("Insufficient product quantity")
+                    });
+                }
+
+                await connection.query(
+                    'UPDATE products SET quantity = quantity + ? WHERE id = ?',
+                    [itemRow.quantity, itemRow.product_id]
+                );
+
+                await connection.query(
+                    'UPDATE products SET quantity = quantity - ? WHERE id = ?',
+                    [quantity, targetProductId]
+                );
+
+                await connection.query(
+                    'UPDATE order_items SET product_id = ?, quantity = ? WHERE id = ?',
+                    [targetProductId, quantity, itemId]
+                );
+
+                await connection.commit();
+                connection.release();
+                return;
+            }
+
             const diff = quantity - itemRow.quantity;
 
             if (diff > 0) {
diff --git a/server/server.js b/server/server.js
index bdf7ee8..4309347 100644
--- a/server/server.js
+++ b/server/server.js
@@ -312,24 +312,28 @@ app.post('/api/orders/:orderId/items', async (req, res) => {
 
 app.put('/api/orders/:orderId/items/:itemId', async (req, res) => {
     let itemId = req.params.itemId;
-    let quantity = req.body.quantity;
+    let { quantity, productId } = req.body;
+    quantity = Number.parseInt(quantity, 10);
     
     try {
         await adapter.updateOrderItem({
             itemId, 
+            productId,
             quantity
         });
         
         res.status(201).json({
             itemId, 
+            productId,
             quantity
         });
         
     } catch (err) {
-        res.json({
+        const statusCode = err.type === DB_USER_ERROR ? 400 : 500;
+        res.status(statusCode).json({
             timeStamp: new Date().toISOString(),
-            statusCode: 500,
-            error: err.message
+            statusCode,
+            error: err.error?.message || err.message
         });
     }
 });