It is far too easy to click on the management screen by accident and either buy something you don't want, or more importantly, sell your best team of horses.
This has happened to me many times. May I suggest different colors for the up\down arrow icons and sell icons? Maybe black arrows and green sell icons, or something similar.
cant say Ive done this myself yet, those buttons are pretty damn large - not very likely to get the wrong one. But yes a single step undo button would be very useful, as long as it wasn't available during the racing.