Contract Law - A promise to pay more for fulfilling an existing contractual commitment owing to the promisor
A promise to pay more than agreed in a contract is not binding unless the promisee provides extra consideration, such as a promise, deed, or omission. Without this, there is no exchange because the promisee is only fulfilling their obligations under the original contract.
Example
Simon is a crew member aboard a sea trip from Southampton to Jamaica. When the boat reaches Jamaica, two crew members resign. However, the remaining crew is experienced and the job on the return journey to Southampton remains unchanged. Simon requests a 50% bonus from the captain in order to return to Southampton. The captain accepts. When they arrive in Southampton, Simon requests a 50% bonus, but the captain refuses. Can Simon sue the captain for his bonus if he worked the return journey to Southampton and offered consideration to make the captain's pledge binding? The answer is "no." Sinbad fulfilled his obligations under the initial deal. According to English law, the captain's promise of a bonus is not binding because he did not provide any consideration.

​additional money for something additional is consideration
Remember that if the promisor under an existing contract promises to
pay the promisee an additional amount in exchange for an additional
promise, act or omission, then this additional promise, act or omission is
consideration. This is because a key element of consideration is
exchange, and this additional promise, act or omission in exchange for
additional money will satisfy this requirement.