How would you deal with being asked to lower your rates?

I've been freelancing doing full-stack web-dev for an agency for the past two months (part-time). I was brought in to help work on a handover project where none of their team members know the programming language in question. Our engagement is now coming to an end, but there is an offer of extending to do a small number of days per month. There is no indication on the number of months.

This arrangement suits me but they want me to reduce my rates by 10%. Any pointers on negotiating on the day rate side?