This.
Ok first of all, it depends on how the company is structured. (do note this does not apply to SME companies)
In some companies, the budget is controlled by the manager, in other companies it is controlled by the HR
But lets say for example, if the budget is controlled by the manager, he will have to think about how is the best way to spend the recruitment before the end of the year/financial year.
Do note the money cannot be brought over to the new year, so any surplus is useless to the manager.
So let's say he is given a budget to spend to hire people and the budget is say 500k (annual salary)
He can choose to hire
2 people at 250k
5 people at 100k
10 people at 50k
etc
Do note he does not need to spend all the money together one shot. Maybe he feels he hire one person at 250k, then keep the rest in case someone resign from the company.
Another thing to note is, just because he can spend to hire 10 people for 50k doesn't mean he can
Because there is another thing called headcount. Basically it means if his team max headcount is 10 people, he cannot have 11 people or more in his team, regardless of whether he has a recruitment budget or not.