You want money? I don't owe you any money.
Letter? What letter? Oh, you wanna fight then? All right, mate... let's do this! Owe you any money…
If you read the description of the quest  , Calvin Montague states "You should check it out... you do and I'll pay you well." There are no monetary rewards from that quest.
Return to Calvin Montague and confront him. He challenges you to fight him. If you win, he surrenders and gives you some money. Otherwise he kills you.
Incredible! What skill!
Thought you'd be easy prey, <name>. Got me good, you did.
Here's the coin I promised... not as much as I said it'd be, but that's on a count o' me lying to ya in the first place.