Praise be to Allah.
A debt which is due now takes precedence over Hajj, because this obligation comes before Hajj. So he should pay off the debt and do Hajj. If he has nothing left after paying off the debt, then he should wait until Allaah makes him independent of means. If it is a long-term debt to be paid off at a future date, and the person feels confident that he will be able to pay it on the agreed date, then in this case the debt does not mean that Hajj is not obligatory for him, whether the lender gives him permission or not. If there is no guarantee that he will be able to pay it off as agreed, then he should wait until the date for the payment comes.
Based on that we say: if a person owes a debt to the mortgage fund, if he knows that when the time comes he will pay his debt, then Hajj is obligatory for him, even if he owes a debt.
Fataawa Ibn ‘Uthaymeen, 21/96
See also the answer to question no. 36852.