I'm not sure about the US, but the UK is providing arms to Ukraine under bilateral aid agreements. The exact terms of these agreements aren't public, but traditionally bilateral aid agreements heavily favour the country giving, as the country receiving is desperate. Thus, the weapons are not being donated, they are being provided as a long term loan.

It's almost inevitable that Ukraine won't be able to pay back this loan. This means the UK will probably have to write off this debt, eventually. However in the meantime the books can be balanced (or at least it makes it look like the government hasn't raped the country as much as it has) and the write off will be a future government's problem. Also, whatever amount Ukraine will still pay should help ensure the country remains on the back foot, should it survive its invasion from Russia.

No real surprise from a government in bed with Russia over the years leading up and into the invasion.

Suffice it to say, the war machine money printer go brrrr.