Some time ago I had a similar idea of providing cash to a lender as collateral to lend back to me. Although I didn’t need a mortgage, I wanted the interest as a deductible. The bank thought it was a wonderful idea, but of course in those days they were falling over backwards to lend under any terms. I think if I’d said my income is derived from selling rocks from Mars, once I’ve built my space ship, they would have giggled delightedly and said “why of course you can have a loan”. Seriously though there are better qualified people here than me on how to structure a loan, and I echo above that you may want to wait a little longer.
