Write an Advanced Drupal 7 Function/s

We are talking about [login to view URL] - a site which is almost complete but has some coding niggles. The website is a SAAS website which is not yet public - but it will manage private loans for people who borrow outside of the banking system. The website is a service system for the lender to keep track of repayments and interest rates and enable them to send reminders and warnings to borrowers and issue statements. I need to completely rewrite/rework a function that changes the interest rate on loans. I have a function that creates a payment schedule when the loan begins but need a function that can change that payment schedule from any interest rate to any other at a specified date in the schedule. The function will need to modify the payment schedule (which is actually one database table per loan) and build a new table based on the modifications. It will then record the change in a separate table so that it can be undone later if required. Once that task is complete there will be some other smaller less complex tasks to help in tidying up the code and finishing the project off. Please allow a full day of work for someone with php/drupal/mysql skills and another half day to full day for the tidying up and finishing off. Once I have an applicant in mind - I'll submit the code to be modified with the changes for reading before you accept the job. This will make sure you are comfortable enough to proceed. The bidder must be mathematically competent and be able to work with decimals, percentages in dates in php and the drupal 7 api.