Musar zu Bamidbar 18:12

כל חלב יצהר וכל חלב תירוש ודגן . "All the best of the new oil, wine, grain, etc." We have the Torah on record here that he who gives the Priests his dues should give them of the best of his respective produce to show that he wishes to fulfill that commandment sincerely. Similarly, when giving charity, [at a time when the coins in circulation sometimes had worn off and their value was somewhat questionable] he should give only coins which are in mint condition.