DEAR ROB: Your recent article on the "residence replacement rule" made me think we should have used that law to save tax when we sold our home in November, 1975, for $43,000 and moved to an apartment. Our profit was about $21,000 on which we paid tax in 1975. In July of 1976 we bought a condomonium apartment for $57,500. Is there any way we could have used that "residence replacement rule?" Bernie N., Oxon Hill.

DEAR BERNIE: Yes, anytime before the Statute of Limitations expires three years after your tax returns were due or were filed, whichever is later, you can amend your 1975 tax returns. Your 1975 tax returns were due on April 15, 1976, so you have until April 14, 1979, to amend your return and get a tax refund. IRS Form 1040X should be used. Attack IRS Form 2119 (Sale of a Residence) showing details of your home sale and replacement.

IRS Code Section 121 requires you to defer the tax when buying a replacement for your principal residence that is of greater value than your former home. The replacement can be bought anytime within 18 months before or after the sale. Condominiums, cooperative apartments, houseboats, and even mobile homes qualify. Both residences must be your principal residence, so vacation homes can't qualify.

DEAR BOB: I gave a 90-day option to a man to buy my home for $62,000. He paid me $10 for the option. Now I realize my home is worth at least $70,000. Can I cancel the option? Mabel Q., Rockville.

DEAR MABEL: AN option is usually valid as long as any valuable consideration was paid for it.In the absence of evidence of fraud or other illegal act, courts usually won't inquire into the adequacy of consideration for an option contract. If that option was recorded, it is now a cloud on the title of your property too. Perhaps your lawyer can find some loophole in the writing of the option.

DEAR BOB: We agree with your advice that it's smart for home buyers to invest as little of their own cash as possible. That's why we want to have the seller take back a second mortgage on the house we buy. Is there any rule of thumb as to how large the monthly payments on a second mortgage should be? Lerner A., Maryland.

DEAR LERNER: The amount of the monthly payment on a second mortgage is negotiable between the borrower-buyer and the lender-seller. In many areas, the payment customarily is 1 per cent month of the original balance.

For example, if you get a $5,000 second mortgage, the monthly payment might be $50. But many second mortgage lenders prefer interest only payments, with the full balance due in five or 10 years. Negotiate the terms with your seller to meet his requirements and yours.

Robert J. Bruss is an attorney and realty broker. For your copy of his report, "How Senior Citizens Can Save Taxes When Selling Their Residences," send 10 cents in coin and a STAMPED self-addressed envelope to P.O. Box 6710, San Francisco, Calif. 94101.