Your credit score will be worth improving to acquire better financing options.

Gain is your net sales price minus your adjusted cost basis (purchase price plus capital improvements minus depreciation). 

@Donald Hatter We send the information to a collection agency who attempts to recover the money; after they make several attempts they place it on their credit report. 

If the property were owner-occupied, would I only receive credit for one rental unit's income, while the non-conforming basement income wouldn't count?

This is much lower than the costs associated with refinancing.Benefits of Mortgage RecastingLower Monthly Payments: The most immediate benefit is a reduced monthly payment, which can free up cash for other financial goals.Save on Interest: By reducing your principal balance, you’ll pay less interest over the life of the loan, potentially saving thousands of dollars.No Credit Check or Appraisal: Unlike refinancing, recasting doesn’t require a credit check, income verification, or home appraisal, making it a quicker and simpler process.Keep Your Current Interest Rate: If you have a favorable interest rate on your current loan, recasting allows you to maintain it while still lowering your payments.Drawbacks of Mortgage RecastingRequires a Lump Sum: Not everyone has the cash on hand to make a significant lump sum payment, which can make recasting inaccessible for some homeowners.Doesn’t Shorten Loan Term: While your monthly payments are lower, recasting doesn’t reduce the length of your loan.

Your son is responsible for making payments on time or ruining this seller's credit. 

Can you pull the cash off the business credit card?

@Justin FondrenIf your credit score is in the 700–730 range, you're in a great position to secure favorable loan terms.

Interest rates and cap rates change, competition changes, your PM ends up being horrible, lots of variables.Just to throw you a monkey wrench, in your killer spreadsheet does it include the credit score, income, and employer of every resident? 

Quote from @Argel Algura: Hey Bigger Pockets Community,A year ago I wrote about rebuilding and starting over in life and considering real estate investing as a path (see post here: "Bad credit, minimal cash and starting over in life . . .