After reading the edits and clarifications and other answers, my suggestion is that you approach this in a similar way to publishing a book in general.
Offer royalties on sales of the book plus an advance.
The way advances works is they're a guarantee of payment, not in addition to royalties. For example, if the advance is $500 and the author has made $1500 in royalties, you pay the $500 up front, then another $1000 over time. If the advance is $500 but the royalties only come out to $400, the author does not have to pay back any part of the advance.
1000 copies of a $100 book online is pretty ambitious (especially if that's in American dollars), but you know your market better than any of us. So let's assume that's reasonable. Out of that $100,000, you only get to keep a portion of it. You mentioned Amazon. Kindle Direct Publishing royalties for ebooks are 35% (with a 70% option if you meet their requirements). Royalty structure is a bit different for other Amazon platforms. If you're selling a print book, it looks like you get more like 25% of the retail price.
To keep the math clean, let's average this out to 50% of retail. Which means a total of $50,000 (again, this is a very high number, but you can replace it with better estimates as you research the market and figure out your publishing strategy).
Some standard royalties to authors are 10% of the retail price or 50% of the publisher's net. These do not calculate the same and there are a lot of "it depends." I've seen a pretty wide range here and some publishers really lowball authors (don't be one of those publishers).
I'm assuming you're doing the translation yourself as a creative endeavor and that you'll be compensated from sales of the book. If you're hiring a translator, then figure that in. So what are your expenses? You'll want a professional editor to go over the translation and there are some services for getting it into ebook format. If you are printing, either you use a print-on-demand service or you have (or are) a printer and have those expenses. Dont' forget postage. Just factor it all in. Do not count overhead or salaries for you or your staff (the reason you don't pay yourself as an expense is because you're going to profit from sales; you can't (shouldn't) do it both ways).
Let's assume your expenses are $6000 (that's high but this is an example) and that you're willing to split your profits 50-50 with the original author (if you have a lot of overhead and/or staff and/or are doing more than translation and reformatting, you might give the author less). That means you expect that you'll earn $22,000 and the author will earn $22,000.
You might want to offer the author a $2200 advance with a contract specifying royalties of 50% (or whatever) of your net. Do not under any circumstances promise any income beyond the advance. Include in your contract that you have the exclusive rights to publish in Ukrainian (make sure you have a lawyer look over the contract and get it just so).
This all assumes that the author is the copyright holder and has retained the rights to translate and republish in other languages. If any of that is not true, you will need to negotiate with the original publisher. If the book was traditionally published, there will be a contract and there will be a clause about translation and republishing.
Larger publishers probably have set fees and standard contracts. They may sell you the rights or they may wish to use you as a subcontractor, assuming they had no previous ability or intent to capture the Ukrainian market. Smaller publishers may have buy-out fees or may want royalties.
You may need to make separate agreements with the author and the original publisher. For example, you might pay the publisher a flat fee then give the author royalties. Remember the calculation for expenses? Fees to publishers go in there. If the publisher wants royalties they will (we hope) split them with the author.
Just be careful that you don't overpromise and that you don't pay fees or advancements you aren't very sure you will make back in sales. 1000 sales of a self-published ebook on Amazon in English is not easy to do. If you're an established publisher in the Ukraine (or in a Ukrainian-speaking community abroad), you'll have a good idea of the sales you can expect, both print and electronic. But if you're taking on the translation task and selling this yourself, you may find sales are a lot less than you hoped.
This is why it's better to offer royalties than a flat fee. It may cost you more if the book does really well. But you pay less upfront and won't lose as much if the book doesn't sell well.