Retainer problem with former client

Hi there, I’m not sure if this is the appropriate group but am hopeful that someone can offer some advice.

A client with whom I’ve been working on a retainer basis (four days each month), emailed me last week to say they no longer needed me and could I invoice them to date. This would’ve been the third week of their month.

I don’t always complete the four days each month as I react to the workload but have been responsive to their needs within half an hour of requests/at weekends/evenings etc.

My original statement of work with them states it as a retainer of four days per month. I’ve emailed them for a final month and they have requested a breakdown of hours spent this month (they don’t normally do this).

I have explained that as it’s a retainer basis, I have been unable to allocate the time to any other clients for this month so will have to charge accordingly (they have given me no notice at all).

They have said they want to continue working with me in the future but needs to justify the time spent. What should I do?? Not only has this retainer come to an end but they clearly don’t want to pay me the full month. I’m new to all of this and not sure what steps to take.

Thanks in advance for your help