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. Under “Contribution”, 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.
For detailed description on how to do this, please read our Knowledge Base article: “Using funds for purchases“.