The Outstanding Mortgage Principal (Box 2) on the 1098 shows the outstanding principal on the mortgage as of January 1st of the tax year the form is reporting.  (Ex. 2020 tax year will show principal balance on January 1, 2020.)  If the mortgage originated during that tax year the mortgage principal is as of the date of origination.  The mortgage origination date is in Box 3 of the 1098.