There was an older couple who had only fifty silver coins left to their name. The man wanted to lend the coins before they were all spent, coin by coin. His wife, a devout Christian, suggested he lend the coins to the church, as God would not only preserve them, but multiply them as well. Together they went to the church and she told her husband to give the coins to the beggars.
Three months passed and they had no money to buy food. The man asked, "When will God repay the debt?" His wife replied, "Go to the church and the money will be where you gave it away." He went straight away, but found no money, nor anyone who would give him fifty silver coins. As he turned to go, he saw one coin on the floor. He picked it up and returned home. "That is fine," said his wife. "Go to the market and buy food. God will repay the rest later."
So off he went to market and bought bread, wine and fish. When his wife cut the fish open, she found a beautiful stone. She had no idea what it was. Neither did he. Still, the man took the stone to a silver merchant, hoping to sell it for a few coins. The merchant asked the price and the man replied, "I'll take whatever it is worth." The merchant offered five silver coins. The man thought that was a lot, perhaps even a joke, so he asked, "Do you really want to pay five coins?" The merchant, thinking his offer had angered the man, offered ten coins. The seller began to think the stone might worth a lot and was silent. The merchant offered 15 silver coins, still getting no response. Then, 30, 40, and finally 50 coins. The man thought, "This stone must be really valuable, or he wouldn't offer 50 coins." So they bargained, finally agreeing on a price of 300 silver coins.
The man returned to his wife, who thought he'd be lucky to get five or ten copper coins. Amazed, she said, "God not only repaid the debt, but with interest, as well." Her husband was so moved by all that had happened, that he, too, became a Christian.
This folk-type story originally appeared in the Russian Orthodox calendar for December 7th, the day after St. Nicholas Day. The story with its reward for the generous poor man who became a Christian is similar to other St. Nicholas miracle stories. By the 14th century it was included with texts for the 6th of December, right along with the other St. Nicholas stories. By the 16th century it had been identified as one of the St. Nicholas miracles, even though the saint doesn't appear.
The vitae of St. Nicholas and His Hagiographical Icons in Russia, Vol. 2, doctoral dissertation by Alexander Boguslawski, University of Kansas, 1980, pp. 129-133.