I'm missing the logic of the second 10k. He had already exceeded the limit. Why did he think that doing it again would change the outcome? It looks to me like he was going to get the extra $25 returned to him and have 10k in bonds. It's definitely vague though.
They thought they exceeded the limit by trying to send $10000+$25, so given that the $10000 was coming back, they expected that they're again $9975 under the limit.
> "A refund of the excess purchase..."
So they'd be refunded the excess, i.e. $25.
Putting another $9,975 through just feels like being "too smart".