[If one said,] ‘‘I take upon myself [to bring] gold’’ — he may not bring less than a dinar of gold. ‘‘Silver’’ — he may not bring less than a dinar of silver. ‘‘Copper’’ — he may not bring less than [the equivalent of] a maah of silver. ‘‘I specified, but I do not know what I specified’’ — he must bring until he [can] say, ‘‘I did not intend this."