Is it permissible to take a joint loan of 800,0 (500,0 with interest from the National Bank and 300,0 without interest from the National Housing Fund), and then take only the 300,0 and leave the 500,0?

A Muslim is not permitted to engage in any transaction that includes both permissible (halal) and impermissible (haram) elements if they cannot be separated. This is because it involves consuming people's wealth unlawfully, and accepting the entire transaction is considered an affirmation of the impermissible.