Don't do it. I think this owner is certainly taking the P..... out of you. First wanting you to reduce your prices by £10.00 per set of nails and then expecting you to replace a set of nails FOC which you didn't do originally.
I had a similar experience a couple of years ago. I went into a Beauty salon that was just starting up. I also paid 20%, and provided all my own products. The problem I has was it was all the owners friends who were coming in for nails originally, and then she would say "Oh, she's a friend, so it's £5.00 discount", so at £25.00 for a set less 20% then another £5.00, I was only earning approx £15.00 per set. They tried telling me I could do a set in an hour, and I told then no, but they still booked then in regardless. Oh, and the days I wasn't there they were messing around with all my products "just having a go", so I just wne in one day and packed my car and told them I was leaving, as their attitude and lack or respect for my products was unacceptable. I had to bin all my acrylic powder and liquid as it could have been contaminated. The owner was not best pleased as she had bookings, but I just told her to sort it out herself.
I have since heard through the grapevine, that the owner did a 2 day course in nails and now has a certificate that covers her for Acrylic, Gel, Fibrelass/silk & tip & dip, not to mention manicure and pedicure - no that's very worrying, as it's taken me 5 years to achieve the relevant qualifications to do the same.
I now work from a small studio in my home and have a regular client base and I'm a lot happier bing my own boss, as I decide the charges and make the rules.
It's very hard to lower prices (unless you offering a special offer for a period of time) and then increase them at a later date.
This is your earnings that she is asking you to take a cut in, and that's not fair - would she half her prices?
If I was you, I would go mobile or set up from home if possible, I think you will find you would be a lot happier than having to deal with all this nonsense.
Good luck