Index: docs/P3-UseCaseModel/UseCase0003.md
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+# Use-case 0003 — Deposit virtual funds
+
+**Initiating actor:** Trader
+
+**Other actors:** —
+
+A logged-in Trader tops up their virtual cash balance. This is a simulation-only operation; no real money changes hands. The operation writes to two tables — the user row and the ledger — inside a single transaction.
+
+## Scenario
+
+1. Trader chooses "Deposit virtual funds" from the authenticated menu.
+2. System prompts for an amount in USD.
+3. Trader enters an amount.
+4. System validates: amount must parse as a positive number.
+5. System opens a transaction and increments the balance:
+
+   ```sql
+   BEGIN;
+
+   UPDATE project.users
+      SET available_balance = available_balance + $1,
+          updated_at        = now()
+    WHERE id = $2;
+
+   INSERT INTO project.transactions (user_id, type, amount, currency, description)
+   VALUES ($2, 'deposit', $1, 'USD', 'Virtual deposit');
+
+   COMMIT;
+   ```
+6. System confirms "Deposited X USD." and returns to the authenticated menu.
+
+### Alternate flow 4a — invalid input
+
+If the amount is non-positive or non-numeric, system responds "Invalid amount." and scenario returns to step 2.
+
+### Verification query
+
+To see the balance after the deposit, the Trader can trigger UC0006, or directly:
+
+```sql
+SELECT available_balance, invested_balance
+  FROM project.users
+ WHERE id = $1;
+```