How do my brother and I compare money sent to our mother with the time spent helping her?

My brother said, "I send Mum a hundred dollars every month. You're making it sound like I do nothing."

I hadn't meant that. He lives several hours away and the money helps. I live nearby and take Mum shopping on four Saturday mornings this month, paying about $25 in petrol myself. She pays for her own groceries and enjoys choosing them in person.

I'd asked whether we could fund delivery or another transport option for one of those Saturdays so I could have a free morning. He heard it as a request to pay me for being her child. I can see why a complaint about time might have sounded like I was dismissing the contribution he can make from where he lives.

Mum hasn't been asked which alternative she'd want, so I don't want to arrange her week over her head. How would you discuss the workload and the money without keeping a sibling scoreboard? I want one Saturday back, not a price for every hour with her.