Is paying the vow amount in installments, with the intention of fulfilling it, sufficient, or must it be paid in one lump sum, especially with the salary doubling?

Your mere statement: "I will donate" is not considered a vow (nadhr) and nothing is binding by it. A vow is only established with a wording that indicates commitment. If you intended it as a vow or uttered a word from the wordings of a vow, then you are obliged to fulfill it, because charity is an act of obedience, and whoever vows an act of obedience is obliged to fulfill it. There is no harm in paying it in installments if it was not vowed to be paid in a single payment.