Want to write stored procedure and test existing code for a billing system

I am looking for a skilled developer to write a stored procedure and test the existing code for our billing system. The existing code is written in C#, so experience with this programming language is a must. Additionally, knowledge of database management and SQL is necessary as we may need to make some changes to facilitate the stored procedure. It is not yet decided if unit tests are required for the existing code, so being flexible and able to adapt to potential changes is important.

There 9 Points where we have to work
Sale
Edit Sale
Sale Return
Edit Sale Return
Purchase
Purchase Edit
Purchase Return
Edit Purchase Return
And Delete For Sale, Purchase