DEAR BOB: I want to sell my house to my son at its fair market value. But I don't want to charge him any interest on his monthly payments to me. My tax man says I must charge my son at least 6 percent interest, He says if I don't, the IRS will "impute" interest at 7 percent. Is this true? Victor B., Oxon Hill.

DEAR VICTOR: Yes. The IRS requires you to charge your buyer at least a 6 percent interest rate on the unpaid balance of the installment sale obligation. Your tax advisor is correct.

DEAR BOB: Our home is listed for sale. An agent from another company brought us a purchase offer which, as one of the conditions, said it expired "upon presentation." We wanted to think the matter over as the price offered was way below our asking price. But the agent said if we didn't accept on the spot, the offer automatically expired. Is this true? Mae A., Bethesda.

DEAR MAE: Yes.I don't think it is a good business practice for agents to write up purchase offers that aren't valid for at least one or two days. But such offers demanding immediate acceptance or rejection are often written. This practice prevents "offer shopping." If you accept such an "upon presentation" offer a day or two later, then the buyer isn't obligated unless he approves your late acceptance. Your attorney can explain further.

DEAR BOB: I sold my six-store commercial building at a big profit. How can I avoid paying tax on that profit? Ray R., Washington.

DEAR RAY: Sorry, but you can't. You could have made a tax-deffered trade for a larger "like-kind" investment property. Since you've already sold, it's too late.

DEAR BOB: In 1978 I bought a small condominium apartment for $47,900 in Florida for my mother. She only has social security income so she can only afford to pay me $25 per monty rent. But she insists on doing this even though my mortgage payment, property tax and monthly condo fee total more than $400 per month. When my CPA did my 1978 income tax returns, she said I couldn't take any depreciation deduction but could deduct the mortgage interest and property taxes. Is this correct? I thought you said that owners of investment property can dedcut depreciation. Norine N., Washington.

DEAR NORINE: Your CPA is correct. A special tax rule applies to non-profit rental of property to relatives at rents that are substantially below market rental value,

In such situations, the owner is entitled only to deduct expenses up to the amount of rent collected. Buy mortgage interest and property taxes are always deductible even if they exceed the rent collected; Since your rent collected is far below your mortgage interest and property tax expenses, you can't take any depreciation deduction.

DEAR BOB: In a recent article you said that Uncle Sam subsidizes home buyers in the form of income tax savings due to itemized deductions for mortgage interest and property taxes. This isn't entirely correct, as any single person can take a $2,200 standard deduction and married couples get a $3,200 tax zero bracket decuction. So taxpayers who buy a home and pay less than $2,200 or $3,200 for their itemized deductions don't get any extra tax benefit. Michael M., Hagerstown, Md.

DEAR MICHAEL: You're partly correct. However, it's pretty hard to buy a home today and pay less than $2,200 to $3,200 per year total for property tax and mortgage interest. Since most homeowners pay more than these amounts, it pays them to itemize their income tax deductions.

For example, suppose you're in a 30 percent income tax bracket and you're buying a home with a 30-year mortgage for $50,000 at 10 percent interest. The itemized interest deduction would be about $5,000 per year, resulting in income tax dollar savings of about $1,500. Another way to look at it is that the true interest is only about 7 percent (10 percent multiplied by 30 percent is 3 percent; 10 percent minus 3 percent is 7 percent).

DEAR BOB: Several years ago we sold our home, on which we had an FHA mortgage. About six months after the sale, the buyer stopped making payments because he lost his job, He stayed in the house about six months before the lender did anything. The first thing we knew about it was when the FHA notified us that the house was sold at a foreclosure sale for $2,354 less than was owned on the mortgage. Now the FHA wants us to pay the $2,354 even though we sold the house two years ago. Can they do this? A. A.

DEAR A. A.: When a VA or FHA borrower sells his residence, and the buyer takes title "subject to" the VA or FHA mortgage, the original borrower remains liable on that loan for any deficiency loss VA or FHA suffers. For this reason, it is important for VA or FHA borrowers to insist that their buyers "assume" the existing VA or FHA mortgage and that VA or FHA releases the seller from further liability. Although it looks to me like you're liable for the deficiency, consult your attorney for advice.