Thankyou for your question, sorry it has taken so long to respond. First and foremost, Diesel fuel is NOT to be put into any Claybased or water based sealer. If you have had a cutback which is a oil based sealer, it may have been put in that, but would not recommend this in any sealer. Diesel is a heavy fuel that will not evaporate as fast as gasoline or mineral spirits. If you are in question as to what was applied to your lot, call a couple quality random sealing contractors to come and look at your lot. They can give you an honest opinion as to what product was applied to your lot. IF DESIEL fuel was added to the sealer you would have a serious tracking problem that is for sure. your floors and sidewalks will be full of sealer marks from foot traffic and tire cutting on the surface of the lot. It would really be noticable on your oil spots where vehicles park and drip fluids.. would be a real mess. As far as the price? You i hope got more than one bid? Always check the references of the bidding contractor, get a list of a few jobs close by and take a ride and look! Sounds like you may have gotten caught up in a fly by night situation. Mabe not! If another contractor can prove the contractor put diesel in his sealer and applied it on your lot, you would be entitled to compensation. But without proof you have to take the word of the contractor you paid.. he has done the work, he has to be paid, and if you are holding his or her money till you get it resolved, the sealing contractor should do whatever it takes to make you comfortable with his work within reason, and then pay the balance. I think a warrantee of any repuatble sealing company should be in force for one year, so look at the lot again in 10 months and have the contractor come and meet you on the lot for satisfaction if anything. Thanks for your comment and question.