Import delta of SQL Table records vs Excel Import File

I have a main table to be populated. Till now the process was that, we had a staging table to which we imported a large Excel file, and from the staging table, only the missing records ('not in' command) in the main table were populating the main table via a stored procedure utilizing the stage. The issue is that database doesn't have any more than 700 MB of space before reaching the license limit for the db. I've been asked to import just the delta into the staging table (i.e only the missing records in the main table on import through Excel) because of disc space constraints. The excel file size is large, therefore excel cannot open it. I cannot import the whole excel file to stage due to disc space constraints. I have to do this via a query, Can anyone here suggest a solution?

Thanks in advance