If a person is very deep in debt and another person upon whom zakat is wajib wants to help him but fears that if he/she gives him zakat he will use it otherwise and not pay off his debt, then the wealthy person should simply seek his permission to pay off his debt. If the poor man permits him and the wealthy person pays off the debt with the intention of discharging his/her zakat, both the zakat and the debt will be considered paid.

(See Idhaahul Masaail by Hazrat Mufti Shabbir Ahmed Qasmi, May Allah preserve him, p 113)