I can think of two ways you can do this, depending on what kind of budget you want. The easiest way is to just categorize the transfers as Income. But if you want the budget to accurately reflect *your* income and *your* spending and not your income plus part of your wife's, I would probably choose to instead divide the money she transfers on the categories you spend that money on.
Here's what I would do:
- Create a budget category called "Temporary holding place" or "Wife's credit card transfers" or something. Don't budget anything into this category - leave it at zero.
- If she transfers $ 500 to the credit card, go to the credit card register and enter an *inflow* of $ 500 in the category you created above.
- At the end of the month, calculate how the money from that credit card has been spent. You can do a rough approximate (for example, you can feel pretty confident that half of your credit card use was Restaurants, a quarter was Entertainment and a quarter was Gas), or you can get the exact numbers from YNAB by filter searching (and I *think* you can use the Reports function for this, too) combined with a calculator to get percentages (Restaurants 52 %, Entertainment 24 %, Gas 22 %, Groceries 2 %) and use those.
- Now, you know her share of this was $ 500, so create a new split transaction where the total amount is zero. In this split transaction, enter an outflow in the Wife's Credit Card Transfer account for $ 500, and enter *inflows* in the affected categories. If you figured out above that 52 % of the credit card usage was in Restaurants, you enter $ 500 * 52 % = $ 260 as an inflow on the Restaurants category. Do the same for other categories until all $ 500 is spent.
When you do this, you will notice that the $ 550 outflow you previously had registered in your Restaurants category has decreased to $ 290. That's not the money you as a couple spend on Restaurants, that's *your* share of the money you spend at restaurants (your share of the money on the credit card plus whatever you have spent on your own). If you pick this way of doing things, it certainly has advantages because you get more correct numbers that way, but keep in mind that if you have budgeted $ 300 for Restaurants, the category will be listed as Overspent for a while until you get around to doing your big transfer transaction (which is so time consuming you won't be doing it more than once a month). It can probably be a good rule of thumb to always make sure your total overspending don't exceed whatever's available in the Wife's Credit Card Transfers category.
That's the way I would have done it... but as I was saying, just categorizing it as Income is easier. A *lot* easier.