If you paid for an item and it is refunded in the same month, your budget will take this into account and add, what was a minus amount. For example, if your budget is for £150 and you paid for a £50 item to take it down to £100 remaining, after the refund your budget would move back to £150.
If you’re refunded from the previous month, your budget will stay the same if you have not spent anything, but if you have spent some of your budget already, the refunded amount will be added onto your budget, only up to the amount set. As an example, you start the month with a £150 budget and you receive a refund, this is not taken into your budget and it remains at £150. If you have already spent some of your budgets, £100 left of £150 and you receive a refund for £60, the budget will move back to £150 and you can bank (as Anne Robinson says) £10.