Hey Jack, when I read this, I wasn't sure what you meant. But I'll try to correct it as good as possible. What do you mean with the second sentence? If you translate it back to English, I'll translate it to Dutch if you like. :)
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Hallo/Beste Danny,
Ik heb een brief van u ontvangen waarin u me vroeg 10 euro te betalen. Dit heb ik op 16 januari betaald.
Met vriendelijke groet,
Jack
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Notes:
"Groet" is only used for saying goodbye to someone, not to say hello. In a informal context, "groetjes" is used mostly to say goodbye, which has the exact same meaning.
In Dutch, we say and write the day before the month, which means we say "27 maart 2015" or in your letter: 16 januari.
I assume this is a formal letter, considering your use of the word "u" instead of "je". When saying "hello" in a formal letter, you can use "Beste Danny" or, when you only know his last name: "Beste meneer ..."/"Geachte meneer ...".
Good job! :)
I'm willing to help you at anytime, just message me if you need some help.