Original PR description

**Steps to reproduce:**
* Install the **Accounting** module.
* Create a customer invoice with at least one product line.
* Manually adjust the **price_unit** on the invoice line.
* Change the fiscal position on the invoice.
* Click the **Update Taxes and Accounts** button.

**Observed behavior:**
* The manually adjusted price is reset to the product’s default sales price (e.g., 1000).
* This occurs even though neither the product nor the UoM changed.
* In contrast, **Sale Orders correctly preserve** manually edited prices in the same situation.

**Cause:**
* `action_update_fpos_values` method call the recomputation of unit price each
  time when we click update taxes and accounts button on invoice.

**Fix:**
* Add a condition to **skip price recomputation** when fiscal position changes.
* This preserves manual prices when only the fiscal position changes.

opw-5252832