Do we make a contract if the amount is very small?

Do we have to make an appendix if the subcontractor still has some days left in his contract but the end date has passed?

If the subcontractor has worked more days than in his subcontract, and we have to make an appendix for a new phase of the project, how many days do we put in his appendix: only the last phase project, or these days + what was worked and not contracted before?