Manual currency converter
Convert to rubles
Write #USD 100 in chat and the bot converts the amount to rubles. Without a target currency it always uses RUB, regardless of the channel currency.
It works with any currency, not only USD. You can find the currency codes list here.

Convert to any currency
To pick the target currency, add it at the end: #USD 100 #EUR.

Converting amounts from regular messages (without #) to the channel currency is described in Automatic Currency Conversion.