All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and messenger.
If this money is a gift from your husband or part of the maintenance that is due upon him for you, then it is your property and falls within your liability as soon as it enters into your possession. The commentary of Al-Kharashi on Mukhtasar Khaleel Al-Maaliki reads: “The well-known opinion of the Maaliki school is that the wife is responsible for all what enters into her possession as a personal right including maintenance, clothing and other rights like suckling wages, and so on, either past or future, i.e. she guarantees it if it is lost, whether she has evidence of the loss or not, whether her husband believed her or not, and whether she caused its damage or not, because she took possession of this as a personal right.” [End of quote]
Based on this, if you save anything from that sum, it is yours, and you may dispose of it in the same manner that you dispose of your own money, so it is permissible for you to pay the debt from it.
The same thing applies to the money that you spend from the money that your husband gifted to you and you possessed; because it has entered into your possession and you may dispose of it as you wish.
However, if it is neither of the above two cases, then, according to principle, it is not permissible for a wife to dispose of the money of her husband without his permission.
'Abdullaah ibn ‘Umar narrated that the Prophet said: “It is not permissible for a wife to give out anything from her household without the permission of her husband." [Abu Daawood and An-Nasaa’i]
Moreover, Abu Umaamah Al-Baahili narrated that the Prophet said in the Farewell Sermon: “A wife is not permitted to give charity from the house of her husband without his permission." [At-Tirmithi]
This is with the exception of minor things that are usually excused according to customs.
For more information on how to deal with a husband who is addicted to marijuana, please refer to Fatwa 86282.
Allaah Knows best.