C.,
It's not really her money. When she pays for her food, shelter, and basic life necessities, then her money is her money to spend however she wants. And if you are coming up with jobs just so she can earn money, well, that's like the summer work program. Fake jobs are created just so kids have a job in the summer. It costs more from others' pockets than the kids are earning. It's got to be real work. Better yet if she earns it from someone else for babysitting, cleaning, etc. and not from you or it's really like you are paying for them if it's more than she normally gets for an allowance.
However, it's just the brand name that she wants, because there are plenty of copy cat boots that look like Uggs at a fraction of the cost. Does she hang around with other kids who have Uggs? Are these kids more affluent? You obviously aren't wealthy or you wouldn't be here asking or saying they're expensive. Now I'm all for quality and balancing that quality with price, but I looked up the Uggs, and they were about $140, right? That IS a lot unless you're well off. I realize there's a lot of leather and stitching to those boots, and they are probably good quality, but.. she has a perfectly good pair that "don't match everything"...Uh, that's a big clue there! :) Make her wear those! (Or sell them to offset new ones?)
How did she get the first two pairs? That's a problem. She's managed to get them twice, so why not now? How old was she when she got the first pair, now outgrown? Were they a gift? Maybe you should just say although you allowed her to have them before, you have changed your mind because it's not necessary to wear designer labels. Or something else you'd rather say.
Does she ask for other designer label clothing? Are you wearing designer labels? Even if you do, you can justify it because you are an adult, paying for your living expenses, and children aren't entitled to all the rights and priveledges of adults...and responsibilities and stress and ......
Good luck! Let us know what you decide and how it turns out. I'm sure you'll get a lot of good advice here to sort through and figure out what might work best for you.