Hi,
To avoid using your credit, you need to stick to the items that are included in the list of free services, and don't exceed the quantity that it says is included. Looking at the list, neither B1ls nor 30GiB Standard SSD show as included. So I would expect the cost of both of those to be subtracted from your $100 credit.
B1s VM is included in the list of free services, as is P6 64GiB Premium SSD, so you could create a VM with those specs instead if you want.
Please go to page below, scroll down, and click "See all free services" button to see the list of included free services.
https://azure.microsoft.com/en-us/free/students/
If the above was useful please click Accept Answer.
Thanks.
-TP