I've always had a problem with matching old ambered polyurethane on repairs where I had to spot-sand the repair. I used Zinssler ambering shellac for small patches, and it worked fairly well on the color match. It was hard to work, though--it took a lot of brushing to keep it from lapping or streaking. Zinssler does not recommend their product for use under polyurethane, either, but I've not had adhesion problems with Mega.

At the paint store where I buy the shellac, a clerk told me to try Zar Amber Varnish stain, #129. I did, and the product works really well. It has heavy pigments in it, but it wipes off in a way that doesn't look paint-like, and it doesn't darken the soft grain, so it matches the natural finish look in that way. I've used it on three jobs now, and the customers were really happy with the match. On at least one of the floors, the orange color was so prominent that I had to add red, yellow, and brown universal tints to the Mega, and apply two coats of that mixture on top of the stain.

The two distributors that I use don't carry Zar, so I have to go out of my way to get the product. I have talked to my local reps from both Duraseal and Bona, and both of them said that it's a problem area they continue to work on. In the meantime, try the Zar the next time you have to deal with this.

The problem is it will match now but not later. The tinted finish or stained area will most likely amber out different than the existing.

That may be true. I've never had a callback on a stain match, so I don't know how they age. I personally would not want to try to sell a customer on the idea that their stain match might amber correctly in six months, but doesn't exactly match right now. People want to see an exact match the week you're doing the job.