You have only broken the quest to retrieve Slasher; it will not effect anything else.
You can keep Slasher if you duplicate it before talking to the orc chief to give it back, or (IIRC) if you have identified Slasher and remove it and any other axes from your inventory, the orc will 'accept' the axe and leave without it.
To copy an item; <span class='standouttext'>Spoiler : </span><span class='spoiler'>quick save the game, then drag an item out of your inventory, but not somewhere there is a red X. Without dropping first, quick load the game, then release the mouse button when it starts to load. This will produce a copy of the item on the ground, with the original still in your inventory.</span> (highlight the text to read)
There is a much easier method for items which can be stacked;
<span class='standouttext'>Spoiler : </span><span class='spoiler'>Drag a stack out of (or into) your inventory (or container) and hold down the control key before dropping. This will bring up the split stack pane; type in a negative number and hit enter (the checkmark will be greyed out). This can also be done in the trade window.</span>
