Praise be to Allaah.
One of the conditions of Hajj is that one should be able to do it, which includes being financially able. Whoever owes a debt and is being asked to repay it in the sense that his creditors will not let him go for Hajj until after he has paid off the debt should not go for Hajj, because he is not able to. But if they are not asking him to repay and he knows that they don't mind, then he may go for Hajj and his Hajj will be valid. It is also permissible to go for Hajj if there is no set time limit for repayment of the debt and he is allowed to repay when it is easy for him. Hajj may be a good thing that may lead to his paying off the debt. And Allaah knows best.
See Fataawa al-Lajnah al-Daa'imah, 11/46; Fataawa Islamiyyah, 2/190.