Rather than change the actual transaction which would be against bookkeeping regs, why not have a deduct from budget field on the transaction until pending has completed?
So it shows £1, you enter £50 below it or whatever the real amount was. The budget page then uses the £50 whilst the transaction is pending, once taken it then looks at the transaction amount instead which should match what you expect it to be.
I tend to add the real amount as a note atm, it would be ace to have a budget pending field to enter instead.