@Hoa Nguyen it depends on your terms. Some HML charge you interest on your max loan amount from day 1 (they call this Dutch interest or loan) others only charge on funds actually drawn. If your loan is interest is Dutch (charged on max loan amount) you should sharpen your pencil and balance the cost of the overestimated rehab costs before closing - make sure you know how much playing it safe is costing you.
Regardless of the type of loan (Dutch or non-Dutch), HML's only provide rehab funds for work completed after some kind of inspection (in-person, photos). They call these draws. The max amount of the draw will be from your agreed rehab budget. Most HML will let you make draws less than budget and some will let you make partial draws (eg you have $6000 for new floors, you draw $3000 against the floors completed in half the property). Most HML charge a fee for each draw. So optimizing your draws will improve your return. If your loan is Dutch, there's no incentive on you to draw less than budgeted (you are already paying interest on the full amount). If it's non-Dutch, you'd typically only draw what you needed (especially if your lender allows partial draws). The other consideration for non-Dutch draws is if you came across an oh-$hit in the project that wasn't budgeted for. In these cases, you can use the full value of your budgeted line items to help offset the out of pocket impact of the oh-$hit.
At the end of the project when you close the sale of the property, you'll have to repay what you borrowed. Any ‘left over' rehab budget, doesn't matter you only repay funds received from the HML.