May the mahr be paid as a debt or in instalments?

Yes. The mahr may be paid in cash, partly in cash and partly as a debt, or entirely as a debt, with the wife's agreement.
A deferred mahr remains a debt owed by the husband that must be settled, and it does not lapse with the passing of time. If the husband dies, the balance of the mahr is paid from his estate before the faraid is distributed. A mahr owed as a debt should be recorded clearly in the marriage register to protect the wife's rights. See Mahr and Faraid.