So I've read a lot about giving source files AFTER the job is finished, but I've started a project with a new client and gave them a PDF of the first draft (presentation deck). However they are asking for the source files right away - I worry that this is a bad move because I haven't been paid yet (they pay at the end) and what if they decide to take what I've done and give it to someone else? They are a marketing firm with plenty of designers on hand. Advice?