All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Whoever intended to give a given amount as Sadaqah (alms) and fails to do so, there is nothing wrong about that. But it is desirable for him to fulfill what he intended seeking reward from Allah. If this is your case, then you can either give your former or current salary. But giving your current salary, which is larger, is better for you.
However, if you vowed to give your first salary as alms and did not do that, then you have committed a sin if you did not have any excuse. So, you should repent to Allah, pay an expiation for breaking an oath for the delay and you must pay a sum equivalent to your first salary; if you give more, then that is better.
Allah knows best.