If you underspend on a budget item, and you have evidence / it's clear it won't happen again, then you just assign that expected spend elsewhere.
Budgets are not fixed for years on end. Monthly or quarterly you would look at your budget, compare actual against estimated and then see if you need to adjust. If you don't have enough, then you need to trim your spend - no pay rises, no temps if someone is away,
Do you get provided with the actual spend against your budget items in real time? You should have been given a template to use by finance.
Then you give it back, they populate with actuals, which gives you the difference in actual spend vs budget.
Always ask for the actual line items which make up the actuals. This way you will see if all spend is yours and finance haven't misallocted, ie coded sometjing to your deot in error, screwed up pre payments / accruals and you can immediately flex - change - your budget to reallocate.
Anyone who can't explain an actual number- real spend- is trying to cover up their own fuck up. Do not assume finance are always right.
Nothing annoys me more than waffle. And babbling on about prepayments and accruals- this doesn't affect your budget, because you would have 100 a month in for rent for example in your budget. You know rent is 1200 a year.
Then actuals would show 100 a month. In the background accounts would journal, pay, whatever, but it's always going to be 100 a month. If it's not, then either you got duff info about rent cost or the bill has gone up and no one told you so you could fuss over your budget and blame finance.