When the time comes to use the money in a fund to purchase the thing you have been saving for, you can do so on the Transaction review screen.  Instead of paying for the item out of your Safe-To-Spend, you choose the fund you want to spend out of.  This reduces the amount in the fund and leaves your Safe-To-Spend undisturbed.  Here’s how.
So, to take the money from a purchase from a fund instead of from your Safe-To-Spend there are two steps.
Select “Fund” from Destination.
Under “Select a fund.” chose the fund that the money should come from.
That’s it!
Also see:
Fund overview screen
How to create a fund