Question: If I do a good act, but with a conscious intention of doing good, does that negate the karma?
Answer: There are two dualistic assumptions buried in your question.
The first is that we can separate ourselves from ‘our’ experience, so there is an ‘I’ acting in ‘The World’. This is counter to our belief that practice is wholehearted action in the present moment, when our ordinary distinctions fall away, vivifying reality.
The second is that our actings take place in linear time, and that good or bad actings in the past have good or bad consequences in the future. But we say that the act and the consequence arise at the same time, the flower and the fruit occur at the same time, and this same time is all of time.