Show him the body of the one we just killed and then return to the mayor to continue our conference.
"So...?"
Assuming that "the one you just killed" referred to the largest one you found, you take a few minutes and lead the miner to the spot where you encountered it.
"Yep. This is it."Hurriedly returning to the entrance of the mines, Likast quietly converses with the mayor.
"Very well, warriors. You have completed that which we asked of you. We can likely survive long enough by now to assemble a military that is capable of contesting the power of whatever lies within the caverns below. We shall present to you the promised reward."The mayor quietly says something to the miner, who hurries off and comes back with a rather large sack.
"We present to you our finest treasures. First, we give you our finest trident. May it serve you well."The miner pulls out the greatest-looking fishing implement you've ever seen. He sets it on the ground.
☼Trident☼ get!"Next, we present you with the most valuable gem we have ever procured."The miner reaches deep into the sack, curses under his breath, and withdraws a sparkling ruby (and a slightly bloody arm.)
Star ruby get!"We give to you three-hundred and eighty golden coins."The miner pulls a pouch out of the sack, shakes it to indicate the fact that there are coins inside of it, drops the ruby from last time into it, and sets it next to the trident.
380☼ get!"Lastly, for your deeds of valor, we present to you our most valuable piece of cultural heritage. From our forefathers in the great halls of Eribaran, to the north, we present to you the greatest weapon forged any dwarf can remember... Ritushang, Beast-Cutter. May you smite darkness with it!"The miner withdraws a greatsword. It is studded with sapphires. It menaces with spikes of steel. It is adorned with rings of an indeterminate type of bone. It is engraved with archaic dwarven runes. The miner drives it into the ground with all of his might, waiting for one of you to claim it.
☼«☼Steel greatsword☼»☼ get!