[nYNAB] How to manage Categories for Loan/Credit payoffs

So I hope I can explain this well.

Let's say i make a purchase for $600 and am given 0% interest for 6 months on credit. So I make a savings category to save $100 for 6 months so in the 6th month that category has $600.

Now in reality that money is just sitting in my savings account and when I make the payment it's a transfer that I cannot assign a category.

So now I have this obsolete category with $600 that doesn't actually exist because the payment has been made via the transfer. Where do I move that money to 0 out this category without it making it appear I have $600 more than I do. Is there some way to make that transfer categorized as that category so I can keep my budget accurate?