der Rubel
ruble
The official unit of currency in Russia and Belarus.
Für die Reise nach Moskau musste er Euro in Rubel umtauschen.
For the trip to Moscow, he had to exchange euros for rubles.

ruble
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ruble
The official unit of currency in Russia and Belarus.
Für die Reise nach Moskau musste er Euro in Rubel umtauschen.
For the trip to Moscow, he had to exchange euros for rubles.

Pay attention to the verb. If the verb is 'rollen' (to roll), it is the idiom 'der Rubel rollt' (business is going well / the money is rolling in). With other verbs such as 'kosten' (to cost) or 'umtauschen' (to exchange), it refers to the currency.
Im Weihnachtsgeschäft rollt bei den Händlern der Rubel.
During the Christmas shopping season, the money is rolling in for retailers.
The expression 'der Rubel rollt' (the money is rolling in) is a fixed idiom. It means that someone is earning a lot of money or that a business is very profitable. The form does not change.
Seit der Eröffnung seines Restaurants rollt bei ihm der Rubel.
Since the opening of his restaurant, the money has been rolling in for him.
The idiom 'der Rubel rollt' (the money is rolling in) is mainly used in colloquial language. You would be unlikely to find it in a formal business report.
Wow, neues Auto? Bei dir scheint der Rubel ja zu rollen!
Wow, new car? Looks like the money is really rolling in for you!
When 'Rubel' (ruble) is used as a currency, it often appears with a number and without an article, or with prepositions such as 'in.' The plural is also 'Rubel' in German.
Er hat 100 Euro in russische Rubel gewechselt.
He exchanged 100 euros for Russian rubles.
money
'das Geld' and 'der Rubel' both belong to the area of payment and currency. 'Geld' is the general term for a means of payment or financial means. 'Rubel' is much more specific: it is a particular currency, for example in Russia or Belarus. The words can be confused because you pay with rubles, and rubles are therefore money. But not all money is rubles. In the expression 'der Rubel rollt' (the money is rolling in), it is also generally about a lot of money being earned.

Lukas hat vor der Reise Euro in Rubel gewechselt.
Before the trip, Lukas exchanged euros for rubles.
Im Urlaub hebt sie am Automaten Geld ab.
On vacation, she withdraws money from the ATM.