It is possible in some cases to sell part of your land when you still have a mortgage on the land.
You would have to get the banks permission to sell part of the land if you have a mortgage on it and the bank would then need to sign the deed to the part of land to the new owner.
So if you bought land using a mortgage and are paying on that mortgage and then want to sell part of the land then you'll need to pay the bank for that part of the land with the proceeds of the sale of the land.
Then the land deed will be transferred to the new owner but if the bank doesn't approve then you will not be able to sell the part of the land when you have a mortgage.
When you have a mortgage you do not own the home or land until the mortgage is paid in full to the bank.
So the bank who you have your mortgage through owns the land or house until you pay back the mortgage in full.