Should we change a reward for top grades when our younger child's improvement earns her nothing?

My daughter put her report beside her brother's and asked, "Does going from a D to a B count?" Under the rule we announced, it didn't. We had promised $10 for each A.

Her brother earned three As and is waiting for the money. She earned none, but had worked through the essay comments with her teacher and moved up two grades in English. I hadn't noticed how the rule would look until she put the reports together.

My partner says changing it now would make a straightforward promise unreliable. I agree we should pay our son what we promised. I also don't want to invent a new payment just because my daughter is upset and call that a fair system.

Could we honour this report's arrangement and replace the rule for the next one? I'm unsure what we would reward instead. "Effort" sounds kinder, but we aren't in every lesson and I don't want the children performing busy-looking study for us.