Can I lower my Student Aid Index by paying down debt or making purchases before filing FAFSA?

Sometimes, legitimately. Because the FAFSA counts reportable assets like cash and taxable investments but ignores consumer debt, using excess cash to pay down credit cards or a mortgage before the filing date can reduce reportable assets and thus your SAI. Similarly, making a necessary purchase you'd make anyway (a needed car, a computer) converts a countable asset into a non-countable one. What you should not do is hide or misrepresent assets, which is fraud. Also remember income is the bigger driver, so asset shuffling only helps at the margin. Time any moves before the day you submit, since the FAFSA is a snapshot. Keep it honest and defensible in case a school asks for verification.