In September, I will have two days available.

I have a couple of prospective parents who have expressed an interest in those days (haven't met them yet, but seem decent on paper).

One of my current mums tonight asked me if she could possibly have one of the days that will become available. The problem is, I really need to fill both days.

Do I turn down my current mum and risk offending her, or do I let her have it and just wait to see if someone comes along who needs the one day?

I do have an unwritten rule of letting my current families have first refusal, but surely that can only work up to a point...