Here’s one of Monzo’s designer’s thoughts on that idea -
Frankly I’d be concerned that your solution would cause quite a few customer support issues, with users forgetting / mistyping the adjustment that they indirectly made to total spend, then wondering why there’s a discrepancy between that figure & the sum of their transactions.
But I totally get the use case & I’m sure that Monzo can figure out a solution, if they change their mind about this.