Rewards are not negotiable as we've enabled you to set your own rates and change them anytime.
For this reason, we expect you to honor the reward at the time you receive a project, since the reward is calculated directly from your Rates.
If you consider the reward to be too low, you can increase your rates at any given time to be reflected on any future project invitations. However, your current invitations will not update the offered reward, since you were invited for that project at a time when your rates were lower.