Agreed-upon rates don't determine the market value of Keys at the time of a sale which is what should be used here.

https://forums.backpack.tf/topic/80085-buying-keys-for-182-each-paypal/ - Unsuccessful buyer at $1.80 (at the time)

https://forums.backpack.tf/topic/80985-buying-keys-182-usd-per-keypaypal-i-can-also-buy-backpacks-sell-keys-safely/ - Successful buyer at $1.82

https://backpack.tf/trust/76561198844438037

https://forums.backpack.tf/topic/82311-buying-keys-usd182-sgd230-no-limits/ - Successful buyer at $1.82

https://backpack.tf/trust/76561198085140336

That'll make the sale 452 Keys which will end up still capping at the buyout of 440 Keys, so the suggestion is fine.

https://gladiator.tf/time-machine?item=It%27s+A+Secret+To+Everybody+Chieftain%27s+Challenge&at=2023-03-23T01%3A51%3A00.000Z

For future suggestions, you need to find threads of successful buyers and/or sellers to determine the price of Keys at the time of a cash trade.