When you create a mission, you have the option to turn on or off a feature for Ambassadors to submit a higher offer than the maximum reward.
Ambassadors can do this when they are projected to have a higher engagement than the maximum reward suggests, and is based on their following and engagement rate.
When an offer is submitted it appears as a request for that mission, where you need to go through their social media to see if their posts have a high enough engagement rate to justify their request. You then have an option to accept, reject, or propose a counter-offer to the user based on what you find.
The reward amount is then agreed upon by both you and the ambassador, and they can send in their mission submission. Only once you have approved their submission, will they receive a higher reward.