Partner receiving extra money for extra work

We are in a partnership with two non-Muslim parties. Our shares in the partnership are 30%, 40% and 30%. Initially, the profit was to be divided as per this ratio when the job was completed. It has now emerged that doing the actual work will require additional time and management. This can either be sourced from an outside consultant or can be carried out by one of the shareholding parties. The other two parties are happy to appoint the third party to do the work for an extra fee. Islamically, is this agreement permissible?