Praise be to Allaah.
You should try to pay off your fathers loan as an act of respect and kindness (birr) to him. Try also to repay only the original amount that your father borrowed, without paying any extra interest, and advise the creditors to take only the original amount. There is nothing wrong with negotiating with them about this, so long as they receive their original loan and they neither treat others unjustly nor are themselves treated unjustly.
Pray for forgiveness and mercy for your deceased father.
Sheikh Muhammed Salih Al-Munajjid