This is a feature / bug that’s been bought up before - although not explained in as much detail as your post 
It sounds like Hugo’s planning to address this when the bill splitting feature is launched, towards the beginning of next year -
I hope you don’t mind me updating the title of your post, original title was “As a user, my spending amount left increases when my contacts pay me”. If you don’t like my edit, please feel free to change it back.