DEAR MR. MELTZER: My husband and I sold our house and made a down payment on a new one. We had promised our daughter that when we sold our house, we would give her our beautiful electric oven. We did this and replaced it with the original oven that came with the house, which had been stored in our basement.

In the meantime, the buyer changed his mind and told us he wanted to back out of the sale and wanted his deposit back. We refused, naturally, because we had already put a downpayment on our new home, and needed the proceeds from the old one to settle on the new house.

The day before settlement was to take place, the buyers visited our old house and noticed the oven had been changed. They said this was not the way the sale had been made, and since we had changed the equipment, they refused to go ahead with settlement. They threatened to sue us for the downpayment.

We told them we would bring back the oven we had given our daughter. But they say it's too late. We need this money to settle for the new house, and we can't find another buyer at this late date. What can we do?

ANSWER: The buyer must settle because when the oven was brought back, the house would be in the original condition in which it was sold. They cannot back out of the deal on that basis. The agreement is valid.

DEAR MR. MELTZER: We kept our aluminum outdoor furniture in the garage all winter. When we took it out and placed it on the terrace, it looked terrible. It's looked that way all summer. It looks old and unattractive, and we bought it only last summer.

Is there anything that can be done to restore its shiny new look?

ANSWER: It's a simple matter to restore the bright new look of aluminum furniture after it's been shut up all season.

Simply take some kitchen aluminum foil, crumple it and rub all over the aluminum parts. You'll be surprised at how wonderful it will look.

DEAR MR. MELTZER: We decided to build a patio onto the back of our house, and so we obtained a contractor's estimate. We told him to go ahead and he said he'd do the job on his first free day.

The following day we spoke with a neighbor about this, who told us that the contractor he had used for a patio was much more reasonable. So we hired him and sent the original contractor a letter, advising that we had changed our minds, and not to go ahead with the patio. Then we left for a holiday.

Upon returning, we found a beautiful patio installed, and we telephoned the second contractor to tell him how pleased we were. He said he hadn't yet been out to our house.

We were shocked! We were even more shocked to receive a bill from the first contractor a few days later. We called him and reminded him that we had cancelled it, but he claimed our letter hadn't arrived in his office until the day after he did the job, which was five days after we mailed the letter.

I can't believe it took six days for a letter to be delivered a few miles. I have refused to pay him more than I would have had to pay the second contractor. What is your opinion?

ANSWER: If you had sent the letter via certified or registered mail, requesting a return receipt, you would have saved yourself money. In that way, you would have proof as to whether he received your letter before the work was done.

This is one case in which someone has tried to take advantage of our slow postal service.

Bernard C. Meltzer is a realtor, engineer and appraiser. He does not answer letters personally but only through this column. His address: Suite 900, 112 S. 16th St., Philadelphia, 19102.