Is it obligatory to donate the value of the financial interest earned from the post office, which was spent on building a house, due to ignorance of the prohibition of interest?

Since you engaged in the transaction out of ignorance of its unlawfulness, then withdrew your money and repented to Allah Almighty, there is no sin upon you. You are not required to dispose of the unlawful interest you used, for Allah Almighty says: "So whoever receives an admonition from his Lord and desists, he may have what is past, and his affair rests with Allah."