A bonus clause is a clause in a contract that rewards the contractor for doing more than the letter of the contract; particularly, to finish the job early. It is in apposition to a penalty clause

Penal damages

Penal damages are best seen as quantitatively excessive liquidated damages and are invalid under the common law. While liquidated damages are a priori calculations of expectation loss under the contract, penal damages go further and seek to penalise a party in some way for breach of a clause above...

 where the contractor loses by providing less than the letter of the contract, or providing it later than agreed.

The aim of a bonus clause is to give a win-win situation whereby the contractor wins by getting more money for finishing early, and the holder of the contract wins by having a product earlier to market.