Why did Malandain accuse Hubert of picking up the lost pocket book in "The Piece of String"?
Maitre Hauchecorne is a relentless miser, always seeking a bargain and seeing value in seemingly worthless items. So, when he spots a piece of discarded string in the road, his naturally thrifty nature compels him--despite a bout of "rheumatism"--to bend over and pick it up out of the dirt. When he looks up to see Malandain watching him, he is embarrassed to be seen by his "enemy" in such a lowly act. Hauchecorne nervously conceals the string and
... then he pretended to be still looking on the ground for something which he did not find, and he went toward the market, his head forward, bent double by his pains.
It is Malandain who later accuses Hauchecorne of stealing a lost purse, the act which sends the old peasant into a downward spiral. Hauchecorne and Malandain are not on the best of terms. A prior business deal concerning a horse's halter had left both of the men dissatisfied, and they are still holding a grudge,
... both being good haters.
So, Malandain has a personal reason to accuse Hauchecorne of the theft of the purse. Malandain could have believed that Hauchecorne was concealing the purse and not the string, believing the old man was hiding something of value, and his accusation could have been faithfully based upon this assumption. But after Hauchecorne has been cleared--the purse has been found and returned to its owner--Malandain "began to laugh" when he saw the peasant pass by. A new rumor (possibly started by Malandain?)
... accused him of having had the pocketbook returned by a confederate, by an accomplice.
We don't know Malandain's true intentions, but based upon the two men's feud; Malandain's false accusation; and the happiness he seemed to feel from Hauchecorne's misery, it does appear that malice was present.