Praise be to Allaah.
Firstly: dealing with interest-based banks is not permissible except in cases of necessity, because it is a kind of cooperation in sin and transgression. Their giving awards to encourage people to deal with them is a kind of encouraging people to do haraam things and comes under the rulings on such things. Allaah says (interpretation of the meaning):
Help you one another in Al-Birr and Al-Taqwa (virtue, righteousness and piety), but do not help one another in sin and transgression [al-Maaidah 5:2]
Secondly: guaranteed profits are not permissible, because the basic principle in shareeah is that.... and that one accepts that if there is the chance of making a profit, there is also the chance of losing. If the government borrows money from the people and gives them a fixed profit, these are interest-based bonds which are forbidden, and it is not permissible to deal with them at all. And Allaah knows best.