Need to track and calculate a changing numeric field

My boss has informed me that from an auditing standpoint, users should not be allowed to change the original order quantity field, but instead we should design a system that logs each numeric change (for example -5 or +9), while also using each value in a transaction log so that the final total (whether + or -) can be added to the original value in order to get the true value. 

Any ideas on simple yet effective ways to implement this? 

Thanks for any help.