Hello Enoch, if you want to use a debt account to track this debt, then you’ll need to adjust your credit card balance by the amount that you’ll have in your debt account. Personally I wouldn’t recommend creating a debt account if this “borrowed” money is actually on a credit card. In my opinion it just creates more work and confusion. I would recommend what Alex said above in his first paragraph, “simply have it as a Credit Card Account in Goodbudget and run the negative balance until you pay it off.”
The reason why I wouldn’t create a separate account on that credit card is because now there’ll be two accounts that will reflect a single credit card account. Just not a good accounting practice.
As for the huge negative balance in your laptop envelope that’s a good thing because it actually shows reality. So what you’ll do is fill that envelope from each paycheck in the amount that will lower that negative balance to zero within the 18 months.
Just a side note on that 0% interest, if you were carrying a balance on that card before you used their 0% interest offer, many card companies apply your monthly payment towards the 0% interest amount, leaving the previous balance earning interest and not being paid down, which is not good for you, but great for the banks. Anyways, that may not be your case, but if you did have a previous balance, you’ll definitely want to read the fine print on how they allocate your monthly payment.