A no spend challenge app with no streak to break
A no spend challenge usually works like a diet: a run of perfect days, a streak counter, and one slip that takes the whole thing back to zero. That is the part people quit at, and it is the part this app does not have.
Why streaks are the wrong instrument
Most goals are not lost to one bad day, they are lost to striving for perfection and then abandoning the whole attempt after breaking it. A counter that resets punishes exactly the person who is doing well but not perfectly, which is nearly everyone.
Keep the Diff has no streak, so there is nothing to break. It counts money instead of consecutive days: every time you decide against a purchase, the amount you did not spend is added up. Miss one, buy the thing, have an expensive week, and the total simply does not grow that time. It never goes backwards.
How a no spend day gets recorded
You log what you were ready to spend and what you actually spent. On a no spend day the second number is zero, which is what the "Didn't buy it" button is for. Ninety dollars planned and nothing spent is a ninety dollar Diff.
Partial counts too, which strict challenges have no way to express. Ninety planned and forty spent is a fifty dollar Diff, and that is a real result rather than a failed day.
What you end up with
A number for the week and the month, a history of the decisions behind it, and optionally a goal to point it at, with a photo of the thing you are saving for. At the end of a month of a no spend challenge you have an amount rather than a badge.
No budget and no bank connection
The app never tells you what you are allowed to spend, and it cannot see what you did spend: there is no bank integration in it. The only numbers it has are the ones you type, and they stay on your device.