Where are you located?
If its bitumin from the backing leave it alone, this is the previous cleaners problem and why didn't they get them back or claim against them.
It could be where a HWE has been carried out with no pre vac, soil has wicked back to surface very common problem with low profile carpet tiles.
Test an area first with MS and charly pad, if it works then set what ever price you charge.I certainly would not offer no result no fee why on earth should you hey have called you in for a corrective clean and should be charged accordingly.