You can't get a refund for the slots, but there is a way to save the slot for later use.
The way to do it is to copy the route to another browser tab.
Then in the original tab, delete the route, don't release slots.
Next, in the new tab, uncheck the day you don't want to fly. You will see that the cost of the slots is zero (since you already own them).
Next, click "confirm".
You will have the new routes with 6 instead of 7 slots, and the 7th slot will be in your posession to reuse for about 2 months. If you re-use it, within this time, either by adding the 7th flight back, or in another route, the cost will be zero.
If you don't use it within 2 months, you will eventually lose it, and you will get some warnings about it, that you are about to lose it.