If I don’t pay an immediate bill, I will be charged interest. Shall I pay that bill off first or the missed Zakat?
One may pay off the outstanding bill. Thereafter, however much money is available, one should endeavour to use the remaining money for missed Zakat even if it does not cover all of the outstanding balance for missed Zakat.
The ideal situation would be to pay the missed Zakat from now in small amounts in instalments in a manner that will not affect the debt payment.