بسم الله الرحمن الرحيم
(1) Your salah shall be valid though it is against the prudence and wisdom to take too many materials on credit in the name of mosque and it is the result of that hastiness which is undesirable.
(2) The construction in-charge should work with the advice of the committee members. In future you should take extra care and all should try to pay the debt off together. Salah shall be valid during this time.
(3) If people donated for the very saf (row) then it is necessary for the construction in-charge to spend it in the same head. And if some amount remains in the fund, he should seek the permission of donors and then spend it in other directions.
Allah knows Best!
Darul Uloom Deoband