Am I sinful for taking the price of a product I did not receive, after finding its value on the invoice, especially since the seller informed me that the price was his responsibility?

If a product is erroneously added to an invoice, the buyer has the right to reclaim its price because it is a trust (amanah) with the seller. Any amount exceeding what is rightfully due to him is considered a trust. There is no harm in insisting on its reclamation. However, it is preferable to be lenient, based on the words of the Messenger of Allah (peace and blessings be upon him): "May Allah have mercy upon a man who is lenient when he sells, when he buys, and when he demands his rights."