At the moment you have to base your scheduled transactions off of transactions that have already occurred (We are working on changing this). That means you are unable to change the value of a scheduled payment at the moment unless you are changing it to a value that you can see on a historic transaction you have made.
Relevant to users on:
Money Dashboard Neon
Not sure which App you are on?
If you are unsure if you are on Money Dashboard Classic or Money Dashboard Neon? You can find that information here