@Daria B.
I wouldn't do the work. I don't ask for upfront payment, per se. I'm not profiting from the material cost. The 10% is nominal and basically covers my time, fuel, and delivery of materials. I've tried having the client provide materials and it seems they always get it wrong..which adds trips to Home Depot for nickel and dime purchases that KILL productivity.
I guess if I went with them, it might work out but I don't want to waste my time while they try to talk me into 2" nails as opposed to 2-1/2" nails...or 1/2" drywall as opposed to 5/8" drywall...or digging through the $0.50 lumber bin trying to find a straight 2x4 to substitute a $2.50 fresh stud. I go into the store, pick up materials, and leave the store. I quote quality materials and if they want me to re-quote with thinner drywall or junk trim to save some money, so be it, but I'd rather be picking it up. I've never tried that approach..
If someone is so stingy that they're not willing to front that cost, they're likely going to be a pain in the butt to work for. I handle myself as a professional, I do professional work, and I expect professionalism from clients. It's just how I do business. It may not be the right way or the normal way, but it's what has worked for me and my clients.
I'm not taking any new work right now because I'm shopping for a flip but that's been my approach in the past.