If a person vows to give a certain amount in charity upon committing a sin, then doubles the amount when fulfilling the vow, is it obligatory to fulfill the first or the second amount?

A vow contingent on a disliked condition is, by scholarly consensus, obligatory to fulfill. If you vowed to give a certain amount in charity upon committing a sin, and you fulfilled it, then nothing further is required of you if you commit the sin again, unless the vow was worded to imply repetition (e.g., "every time"). If you explicitly stated an increase in the vowed amount, then you are required to give that increased amount in charity upon repeating the sin. However, if the increase was merely an intention and not verbally expressed, then nothing is required of you, as a vow is not binding by mere intention.