Agree with hattyyellow, plus some other points
Would be very difficult and very unusual to be able to pinch money from the salaries budget to cover overspend on recruitment budget, it just doesn't work like that, particularly in a big charity which I think this is. Budgets are very tightly managed and planned and you can't just move money about as and when.
Recruitment budgeting is extremely difficult for obvious reasons. You can do a certain amount of forecasting with historical turnover figures, forward knowledge of any 'events' in the coming year which may affect recruitment needs, any restructurings, research about advertising and agency costs, (together with intense negotiation of these) and can even pinpoint significant and expensive posts which you hvae an inkling might become vacant. But the bottom line is you can never know how much recruitment you will need to do.
I have to say I would never authorise payment of first interview expenses on a routine basis as an appropriate or necessary spend of a recruitment budget. Recruiting for charities in my fairly extensive experience involves a careful balance between spending enough on advertising/agency fees/other recruitment methods to ensure the right candidates are reached, but not spending more than is strictly necessary to do this. Charities very often do not compare favourably with either private or public sector organisations in terms of salaries and benefits, but to a greater or lesser degree, a lot of them are fortunate enough to be able to recruit good people in spite of this, there often being a vocational element to people's decision to opt for the voluntary sector.
The same principle applies to things like interview expenses - it's just not necessary to pay them to get good candidates, so doing it as a matter of routine I would consider extravagant (leaving aside the specific example of bidding for government/lottery/other funds for an individual project). I would be more inclined to consider such a request on an individual basis. If a post was proving difficult to recruit to, or candidates were having to spend excessively for one reason or another, I would certainly try to authorise it with the long term view that it might save on further recruitment costs. But I've never had the luxury of a generous enough recruitment budget that this kind of expense could be paid as a matter of routine.
In addition to that, if you think about how much money that actually is. Say, 10 candidates to first interview at £100 each, then 5 candidates to second interview at £100 each, that's £1,500 on interview expenses alone for each post recruited. There would certainly be a strong argument in an organisation running tight on salaries that that money would be better spent on the salary of the person employed.
However we digress into a more general discussion about charity funding, which, although interesting, doesn't help the OP who is going to her interview regardless!
Best of luck